diff --git a/Libraries/LibWeb/CSS/CSSImageValue.cpp b/Libraries/LibWeb/CSS/CSSImageValue.cpp
index d175c8ea32..2438a748e8 100644
--- a/Libraries/LibWeb/CSS/CSSImageValue.cpp
+++ b/Libraries/LibWeb/CSS/CSSImageValue.cpp
@@ -32,7 +32,7 @@ void CSSImageValue::initialize(JS::Realm& realm)
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#stylevalue-serialization
-WebIDL::ExceptionOr<String> CSSImageValue::to_string() const
+WebIDL::ExceptionOr<Utf16String> CSSImageValue::to_string() const
 {
     // AD-HOC: The spec doesn't say how to serialize this, as it's intentionally a black box.
     //         We just rely on CSSStyleValue serializing its held StyleValue.
diff --git a/Libraries/LibWeb/CSS/CSSImageValue.h b/Libraries/LibWeb/CSS/CSSImageValue.h
index 6c40f75fc9..7c563db8e2 100644
--- a/Libraries/LibWeb/CSS/CSSImageValue.h
+++ b/Libraries/LibWeb/CSS/CSSImageValue.h
@@ -20,7 +20,7 @@ public:
 
     virtual ~CSSImageValue() override = default;
 
-    virtual WebIDL::ExceptionOr<String> to_string() const override;
+    virtual WebIDL::ExceptionOr<Utf16String> to_string() const override;
     virtual WebIDL::ExceptionOr<NonnullRefPtr<StyleValue const>> create_an_internal_representation(PropertyNameAndID const&, PerformTypeCheck) const override;
 
 private:
diff --git a/Libraries/LibWeb/CSS/CSSKeywordValue.cpp b/Libraries/LibWeb/CSS/CSSKeywordValue.cpp
index 0881f198fe..fc31ea2b7a 100644
--- a/Libraries/LibWeb/CSS/CSSKeywordValue.cpp
+++ b/Libraries/LibWeb/CSS/CSSKeywordValue.cpp
@@ -5,7 +5,6 @@
  */
 
 #include "CSSKeywordValue.h"
-#include <AK/StringBuilder.h>
 #include <LibWeb/Bindings/CSSKeywordValue.h>
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/CSS/Keyword.h>
@@ -61,19 +60,19 @@ WebIDL::ExceptionOr<void> CSSKeywordValue::set_value(FlyString value)
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#keywordvalue-serialization
-void CSSKeywordValue::serialize(StringBuilder& builder) const
+void CSSKeywordValue::serialize(Utf16StringBuilder& builder) const
 {
     // To serialize a CSSKeywordValue this:
     // 1. Return this’s value internal slot.
     // AD-HOC: Serialize it as an identifier. Spec issue: https://github.com/w3c/csswg-drafts/issues/12545
-    serialize_an_identifier(builder, m_value);
+    builder.append(Utf16String::from_utf8_without_validation(serialize_an_identifier(m_value)));
 }
 
-WebIDL::ExceptionOr<String> CSSKeywordValue::to_string() const
+WebIDL::ExceptionOr<Utf16String> CSSKeywordValue::to_string() const
 {
-    StringBuilder builder;
+    Utf16StringBuilder builder;
     serialize(builder);
-    return builder.to_string_without_validation();
+    return builder.to_string();
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#create-an-internal-representation
diff --git a/Libraries/LibWeb/CSS/CSSKeywordValue.h b/Libraries/LibWeb/CSS/CSSKeywordValue.h
index 80c87ce8f1..e1968cdad9 100644
--- a/Libraries/LibWeb/CSS/CSSKeywordValue.h
+++ b/Libraries/LibWeb/CSS/CSSKeywordValue.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/FlyString.h>
+#include <AK/Utf16StringBuilder.h>
 #include <LibWeb/CSS/CSSStyleValue.h>
 
 namespace Web::CSS {
@@ -28,8 +29,8 @@ public:
     FlyString const& value() const { return m_value; }
     WebIDL::ExceptionOr<void> set_value(FlyString value);
 
-    void serialize(StringBuilder&) const;
-    virtual WebIDL::ExceptionOr<String> to_string() const override;
+    void serialize(Utf16StringBuilder&) const;
+    virtual WebIDL::ExceptionOr<Utf16String> to_string() const override;
     virtual WebIDL::ExceptionOr<NonnullRefPtr<StyleValue const>> create_an_internal_representation(PropertyNameAndID const&, PerformTypeCheck) const override;
 
 private:
diff --git a/Libraries/LibWeb/CSS/CSSMathClamp.cpp b/Libraries/LibWeb/CSS/CSSMathClamp.cpp
index ad71ec8096..2326d4898f 100644
--- a/Libraries/LibWeb/CSS/CSSMathClamp.cpp
+++ b/Libraries/LibWeb/CSS/CSSMathClamp.cpp
@@ -68,16 +68,16 @@ void CSSMathClamp::visit_edges(Visitor& visitor)
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssmathvalue
-void CSSMathClamp::serialize_math_value(StringBuilder& s, Nested, Parens) const
+void CSSMathClamp::serialize_math_value(Utf16StringBuilder& s, Nested, Parens) const
 {
     // AD-HOC: The spec is missing serialization rules for CSSMathClamp: https://github.com/w3c/css-houdini-drafts/issues/1152
-    s.append("clamp("sv);
+    s.append_ascii("clamp("sv);
     m_lower->serialize(s, { .nested = true, .parenless = true });
-    s.append(", "sv);
+    s.append_ascii(", "sv);
     m_value->serialize(s, { .nested = true, .parenless = true });
-    s.append(", "sv);
+    s.append_ascii(", "sv);
     m_upper->serialize(s, { .nested = true, .parenless = true });
-    s.append(')');
+    s.append_ascii(')');
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#dom-cssmathclamp-lower
diff --git a/Libraries/LibWeb/CSS/CSSMathClamp.h b/Libraries/LibWeb/CSS/CSSMathClamp.h
index d5f22d87ad..ed4cb74607 100644
--- a/Libraries/LibWeb/CSS/CSSMathClamp.h
+++ b/Libraries/LibWeb/CSS/CSSMathClamp.h
@@ -28,7 +28,7 @@ public:
     GC::Ref<CSSNumericValue> value() const;
     GC::Ref<CSSNumericValue> upper() const;
 
-    virtual void serialize_math_value(StringBuilder&, Nested, Parens) const override;
+    virtual void serialize_math_value(Utf16StringBuilder&, Nested, Parens) const override;
     virtual bool is_equal_numeric_value(GC::Ref<CSSNumericValue> other) const override;
     virtual Optional<SumValue> create_a_sum_value() const override;
 
diff --git a/Libraries/LibWeb/CSS/CSSMathInvert.cpp b/Libraries/LibWeb/CSS/CSSMathInvert.cpp
index d2607a3907..1f9b8b379b 100644
--- a/Libraries/LibWeb/CSS/CSSMathInvert.cpp
+++ b/Libraries/LibWeb/CSS/CSSMathInvert.cpp
@@ -54,7 +54,7 @@ void CSSMathInvert::visit_edges(Visitor& visitor)
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssmathvalue
-void CSSMathInvert::serialize_math_value(StringBuilder& s, Nested nested, Parens parens) const
+void CSSMathInvert::serialize_math_value(Utf16StringBuilder& s, Nested nested, Parens parens) const
 {
     // NB: Only steps 1 and 6 apply here.
     // 1. Let s initially be the empty string.
@@ -65,21 +65,21 @@ void CSSMathInvert::serialize_math_value(StringBuilder& s, Nested nested, Parens
         //    otherwise, append "calc(" to s.
         if (parens == Parens::With) {
             if (nested == Nested::Yes) {
-                s.append('(');
+                s.append_ascii('(');
             } else {
-                s.append("calc("sv);
+                s.append_ascii("calc("sv);
             }
         }
 
         // 2. Append "1 / " to s.
-        s.append("1 / "sv);
+        s.append_ascii("1 / "sv);
 
         // 3. Serialize this’s value internal slot with nested set to true, and append the result to s.
         m_value->serialize(s, { .nested = true });
 
         // 4. If paren-less is false, append ")" to s,
         if (parens == Parens::With)
-            s.append(')');
+            s.append_ascii(')');
 
         // 5. Return s.
     }
diff --git a/Libraries/LibWeb/CSS/CSSMathInvert.h b/Libraries/LibWeb/CSS/CSSMathInvert.h
index 93195a57d7..7c635f6ec5 100644
--- a/Libraries/LibWeb/CSS/CSSMathInvert.h
+++ b/Libraries/LibWeb/CSS/CSSMathInvert.h
@@ -26,7 +26,7 @@ public:
 
     GC::Ref<CSSNumericValue> value() const;
 
-    virtual void serialize_math_value(StringBuilder&, Nested, Parens) const override;
+    virtual void serialize_math_value(Utf16StringBuilder&, Nested, Parens) const override;
     virtual bool is_equal_numeric_value(GC::Ref<CSSNumericValue> other) const override;
     virtual Optional<SumValue> create_a_sum_value() const override;
 
diff --git a/Libraries/LibWeb/CSS/CSSMathMax.cpp b/Libraries/LibWeb/CSS/CSSMathMax.cpp
index 58e3f9961c..15bedb1a17 100644
--- a/Libraries/LibWeb/CSS/CSSMathMax.cpp
+++ b/Libraries/LibWeb/CSS/CSSMathMax.cpp
@@ -86,7 +86,7 @@ void CSSMathMax::visit_edges(Visitor& visitor)
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssmathvalue
-void CSSMathMax::serialize_math_value(StringBuilder& s, Nested, Parens) const
+void CSSMathMax::serialize_math_value(Utf16StringBuilder& s, Nested, Parens) const
 {
     // NB: Only steps 1 and 2 apply here.
     // 1. Let s initially be the empty string.
@@ -94,7 +94,7 @@ void CSSMathMax::serialize_math_value(StringBuilder& s, Nested, Parens) const
     // 2. If this is a CSSMathMin or CSSMathMax:
     {
         // 1. Append "min(" or "max(" to s, as appropriate.
-        s.append("max("sv);
+        s.append_ascii("max("sv);
 
         // 2. For each arg in this’s values internal slot, serialize arg with nested and paren-less both true, and
         //    append the result to s, appending a ", " between successive values.
@@ -103,13 +103,13 @@ void CSSMathMax::serialize_math_value(StringBuilder& s, Nested, Parens) const
             if (first) {
                 first = false;
             } else {
-                s.append(", "sv);
+                s.append_ascii(", "sv);
             }
             arg->serialize(s, { .nested = true, .parenless = true });
         }
 
         // 3. Append ")" to s and return s.
-        s.append(')');
+        s.append_ascii(')');
     }
 }
 
diff --git a/Libraries/LibWeb/CSS/CSSMathMax.h b/Libraries/LibWeb/CSS/CSSMathMax.h
index d6de4fa4b4..b02df0e1f2 100644
--- a/Libraries/LibWeb/CSS/CSSMathMax.h
+++ b/Libraries/LibWeb/CSS/CSSMathMax.h
@@ -27,7 +27,7 @@ public:
 
     GC::Ref<CSSNumericArray> values() const;
 
-    virtual void serialize_math_value(StringBuilder&, Nested, Parens) const override;
+    virtual void serialize_math_value(Utf16StringBuilder&, Nested, Parens) const override;
     virtual bool is_equal_numeric_value(GC::Ref<CSSNumericValue> other) const override;
     virtual Optional<SumValue> create_a_sum_value() const override;
 
diff --git a/Libraries/LibWeb/CSS/CSSMathMin.cpp b/Libraries/LibWeb/CSS/CSSMathMin.cpp
index 886f62f664..e68dea919c 100644
--- a/Libraries/LibWeb/CSS/CSSMathMin.cpp
+++ b/Libraries/LibWeb/CSS/CSSMathMin.cpp
@@ -87,7 +87,7 @@ void CSSMathMin::visit_edges(Visitor& visitor)
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssmathvalue
-void CSSMathMin::serialize_math_value(StringBuilder& s, Nested, Parens) const
+void CSSMathMin::serialize_math_value(Utf16StringBuilder& s, Nested, Parens) const
 {
     // NB: Only steps 1 and 2 apply here.
     // 1. Let s initially be the empty string.
@@ -95,7 +95,7 @@ void CSSMathMin::serialize_math_value(StringBuilder& s, Nested, Parens) const
     // 2. If this is a CSSMathMin or CSSMathMax:
     {
         // 1. Append "min(" or "max(" to s, as appropriate.
-        s.append("min("sv);
+        s.append_ascii("min("sv);
 
         // 2. For each arg in this’s values internal slot, serialize arg with nested and paren-less both true, and
         //    append the result to s, appending a ", " between successive values.
@@ -104,13 +104,13 @@ void CSSMathMin::serialize_math_value(StringBuilder& s, Nested, Parens) const
             if (first) {
                 first = false;
             } else {
-                s.append(", "sv);
+                s.append_ascii(", "sv);
             }
             arg->serialize(s, { .nested = true, .parenless = true });
         }
 
         // 3. Append ")" to s and return s.
-        s.append(')');
+        s.append_ascii(')');
     }
 }
 
diff --git a/Libraries/LibWeb/CSS/CSSMathMin.h b/Libraries/LibWeb/CSS/CSSMathMin.h
index 8698aeb46d..a2a7d198d7 100644
--- a/Libraries/LibWeb/CSS/CSSMathMin.h
+++ b/Libraries/LibWeb/CSS/CSSMathMin.h
@@ -27,7 +27,7 @@ public:
 
     GC::Ref<CSSNumericArray> values() const;
 
-    virtual void serialize_math_value(StringBuilder&, Nested, Parens) const override;
+    virtual void serialize_math_value(Utf16StringBuilder&, Nested, Parens) const override;
     virtual bool is_equal_numeric_value(GC::Ref<CSSNumericValue> other) const override;
     virtual Optional<SumValue> create_a_sum_value() const override;
 
diff --git a/Libraries/LibWeb/CSS/CSSMathNegate.cpp b/Libraries/LibWeb/CSS/CSSMathNegate.cpp
index 2b38bd20a1..c3719ec669 100644
--- a/Libraries/LibWeb/CSS/CSSMathNegate.cpp
+++ b/Libraries/LibWeb/CSS/CSSMathNegate.cpp
@@ -51,7 +51,7 @@ void CSSMathNegate::visit_edges(Visitor& visitor)
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssmathvalue
-void CSSMathNegate::serialize_math_value(StringBuilder& s, Nested nested, Parens parens) const
+void CSSMathNegate::serialize_math_value(Utf16StringBuilder& s, Nested nested, Parens parens) const
 {
     // NB: Only steps 1 and 4 apply here.
     // 1. Let s initially be the empty string.
@@ -62,21 +62,21 @@ void CSSMathNegate::serialize_math_value(StringBuilder& s, Nested nested, Parens
         //    otherwise, append "calc(" to s.
         if (parens == Parens::With) {
             if (nested == Nested::Yes) {
-                s.append('(');
+                s.append_ascii('(');
             } else {
-                s.append("calc("sv);
+                s.append_ascii("calc("sv);
             }
         }
 
         // 2. Append "-" to s.
-        s.append('-');
+        s.append_ascii('-');
 
         // 3. Serialize this’s value internal slot with nested set to true, and append the result to s.
         m_value->serialize(s, { .nested = true });
 
         // 4. If paren-less is false, append ")" to s,
         if (parens == Parens::With)
-            s.append(')');
+            s.append_ascii(')');
 
         // 5. Return s.
     }
diff --git a/Libraries/LibWeb/CSS/CSSMathNegate.h b/Libraries/LibWeb/CSS/CSSMathNegate.h
index 3acc816403..7dbd503d3d 100644
--- a/Libraries/LibWeb/CSS/CSSMathNegate.h
+++ b/Libraries/LibWeb/CSS/CSSMathNegate.h
@@ -26,7 +26,7 @@ public:
 
     GC::Ref<CSSNumericValue> value() const;
 
-    virtual void serialize_math_value(StringBuilder&, Nested, Parens) const override;
+    virtual void serialize_math_value(Utf16StringBuilder&, Nested, Parens) const override;
     virtual bool is_equal_numeric_value(GC::Ref<CSSNumericValue> other) const override;
     virtual Optional<SumValue> create_a_sum_value() const override;
 
diff --git a/Libraries/LibWeb/CSS/CSSMathProduct.cpp b/Libraries/LibWeb/CSS/CSSMathProduct.cpp
index 92e80ee194..605bb01a31 100644
--- a/Libraries/LibWeb/CSS/CSSMathProduct.cpp
+++ b/Libraries/LibWeb/CSS/CSSMathProduct.cpp
@@ -86,7 +86,7 @@ void CSSMathProduct::visit_edges(Visitor& visitor)
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssmathvalue
-void CSSMathProduct::serialize_math_value(StringBuilder& s, Nested nested, Parens parens) const
+void CSSMathProduct::serialize_math_value(Utf16StringBuilder& s, Nested nested, Parens parens) const
 {
     // NB: Only steps 1 and 5 apply here.
     // 1. Let s initially be the empty string.
@@ -97,9 +97,9 @@ void CSSMathProduct::serialize_math_value(StringBuilder& s, Nested nested, Paren
         //    otherwise, append "calc(" to s.
         if (parens == Parens::With) {
             if (nested == Nested::Yes) {
-                s.append('(');
+                s.append_ascii('(');
             } else {
-                s.append("calc("sv);
+                s.append_ascii("calc("sv);
             }
         }
 
@@ -117,20 +117,20 @@ void CSSMathProduct::serialize_math_value(StringBuilder& s, Nested nested, Paren
             // 1. If arg is a CSSMathInvert, append " / " to s, then serialize arg’s value internal slot with nested
             //    set to true, and append the result to s.
             if (auto* invert = as_if<CSSMathInvert>(*arg)) {
-                s.append(" / "sv);
+                s.append_ascii(" / "sv);
                 invert->value()->serialize(s, { .nested = true });
             }
 
             // 2. Otherwise, append " * " to s, then serialize arg with nested set to true, and append the result to s.
             else {
-                s.append(" * "sv);
+                s.append_ascii(" * "sv);
                 arg->serialize(s, { .nested = true });
             }
         }
 
         // 4. If paren-less is false, append ")" to s,
         if (parens == Parens::With)
-            s.append(')');
+            s.append_ascii(')');
 
         // 5. Return s.
     }
diff --git a/Libraries/LibWeb/CSS/CSSMathProduct.h b/Libraries/LibWeb/CSS/CSSMathProduct.h
index 57ebb305d4..69e5705f08 100644
--- a/Libraries/LibWeb/CSS/CSSMathProduct.h
+++ b/Libraries/LibWeb/CSS/CSSMathProduct.h
@@ -27,7 +27,7 @@ public:
 
     GC::Ref<CSSNumericArray> values() const;
 
-    virtual void serialize_math_value(StringBuilder&, Nested, Parens) const override;
+    virtual void serialize_math_value(Utf16StringBuilder&, Nested, Parens) const override;
     virtual bool is_equal_numeric_value(GC::Ref<CSSNumericValue> other) const override;
     virtual Optional<SumValue> create_a_sum_value() const override;
 
diff --git a/Libraries/LibWeb/CSS/CSSMathSum.cpp b/Libraries/LibWeb/CSS/CSSMathSum.cpp
index 4db823068b..e6bbfb0cba 100644
--- a/Libraries/LibWeb/CSS/CSSMathSum.cpp
+++ b/Libraries/LibWeb/CSS/CSSMathSum.cpp
@@ -84,7 +84,7 @@ void CSSMathSum::visit_edges(Visitor& visitor)
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssmathvalue
-void CSSMathSum::serialize_math_value(StringBuilder& s, Nested nested, Parens parens) const
+void CSSMathSum::serialize_math_value(Utf16StringBuilder& s, Nested nested, Parens parens) const
 {
     // NB: Only steps 1 and 3 apply here.
     // 1. Let s initially be the empty string.
@@ -95,9 +95,9 @@ void CSSMathSum::serialize_math_value(StringBuilder& s, Nested nested, Parens pa
         //    otherwise, append "calc(" to s.
         if (parens == Parens::With) {
             if (nested == Nested::Yes) {
-                s.append('(');
+                s.append_ascii('(');
             } else {
-                s.append("calc("sv);
+                s.append_ascii("calc("sv);
             }
         }
 
@@ -116,20 +116,20 @@ void CSSMathSum::serialize_math_value(StringBuilder& s, Nested nested, Parens pa
             // 1. If arg is a CSSMathNegate, append " - " to s, then serialize arg’s value internal slot with nested
             //    set to true, and append the result to s.
             if (auto* negate = as_if<CSSMathNegate>(*arg)) {
-                s.append(" - "sv);
+                s.append_ascii(" - "sv);
                 negate->value()->serialize(s, { .nested = true });
             }
 
             // 2. Otherwise, append " + " to s, then serialize arg with nested set to true, and append the result to s.
             else {
-                s.append(" + "sv);
+                s.append_ascii(" + "sv);
                 arg->serialize(s, { .nested = true });
             }
         }
 
         // 4. If paren-less is false, append ")" to s,
         if (parens == Parens::With)
-            s.append(')');
+            s.append_ascii(')');
 
         // 5. Return s.
     }
diff --git a/Libraries/LibWeb/CSS/CSSMathSum.h b/Libraries/LibWeb/CSS/CSSMathSum.h
index 84f3fce027..747f4c76b4 100644
--- a/Libraries/LibWeb/CSS/CSSMathSum.h
+++ b/Libraries/LibWeb/CSS/CSSMathSum.h
@@ -27,7 +27,7 @@ public:
 
     GC::Ref<CSSNumericArray> values() const;
 
-    virtual void serialize_math_value(StringBuilder&, Nested, Parens) const override;
+    virtual void serialize_math_value(Utf16StringBuilder&, Nested, Parens) const override;
     virtual bool is_equal_numeric_value(GC::Ref<CSSNumericValue> other) const override;
     virtual Optional<SumValue> create_a_sum_value() const override;
 
diff --git a/Libraries/LibWeb/CSS/CSSMathValue.h b/Libraries/LibWeb/CSS/CSSMathValue.h
index aa715a32e7..c603253986 100644
--- a/Libraries/LibWeb/CSS/CSSMathValue.h
+++ b/Libraries/LibWeb/CSS/CSSMathValue.h
@@ -30,7 +30,7 @@ public:
         With,
         Without,
     };
-    virtual void serialize_math_value(StringBuilder&, Nested, Parens) const = 0;
+    virtual void serialize_math_value(Utf16StringBuilder&, Nested, Parens) const = 0;
 
     virtual WebIDL::ExceptionOr<NonnullRefPtr<StyleValue const>> create_an_internal_representation(PropertyNameAndID const&, PerformTypeCheck) const final override;
 
diff --git a/Libraries/LibWeb/CSS/CSSNumericValue.cpp b/Libraries/LibWeb/CSS/CSSNumericValue.cpp
index e62bb28917..23b633e6d3 100644
--- a/Libraries/LibWeb/CSS/CSSNumericValue.cpp
+++ b/Libraries/LibWeb/CSS/CSSNumericValue.cpp
@@ -5,7 +5,6 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include <AK/StringBuilder.h>
 #include <LibWeb/Bindings/CSSNumericValue.h>
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/CSS/CSSMathInvert.h>
@@ -404,7 +403,7 @@ Bindings::CSSNumericType CSSNumericValue::type_for_bindings() const
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssnumericvalue
-void CSSNumericValue::serialize(StringBuilder& builder, SerializationParams const& params) const
+void CSSNumericValue::serialize(Utf16StringBuilder& builder, SerializationParams const& params) const
 {
     // To serialize a CSSNumericValue this, given an optional minimum, a numeric value, and optional maximum, a numeric value:
     // 1. If this is a CSSUnitValue, serialize a CSSUnitValue from this, passing minimum and maximum. Return the result.
@@ -419,11 +418,11 @@ void CSSNumericValue::serialize(StringBuilder& builder, SerializationParams cons
         params.parenless ? CSSMathValue::Parens::Without : CSSMathValue::Parens::With);
 }
 
-String CSSNumericValue::to_string(SerializationParams const& params) const
+Utf16String CSSNumericValue::to_string(SerializationParams const& params) const
 {
-    StringBuilder builder;
+    Utf16StringBuilder builder;
     serialize(builder, params);
-    return builder.to_string_without_validation();
+    return builder.to_string();
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#rectify-a-numberish-value
diff --git a/Libraries/LibWeb/CSS/CSSNumericValue.h b/Libraries/LibWeb/CSS/CSSNumericValue.h
index 5c8c8d4c77..13487fdc4e 100644
--- a/Libraries/LibWeb/CSS/CSSNumericValue.h
+++ b/Libraries/LibWeb/CSS/CSSNumericValue.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/FlyString.h>
+#include <AK/Utf16StringBuilder.h>
 #include <LibWeb/Bindings/CSSNumericValue.h>
 #include <LibWeb/CSS/CSSStyleValue.h>
 #include <LibWeb/CSS/NumericType.h>
@@ -56,9 +57,9 @@ public:
     Bindings::CSSNumericType type_for_bindings() const;
     NumericType const& type() const { return m_type; }
 
-    virtual WebIDL::ExceptionOr<String> to_string() const final override { return to_string({}); }
-    void serialize(StringBuilder&, SerializationParams const&) const;
-    String to_string(SerializationParams const&) const;
+    virtual WebIDL::ExceptionOr<Utf16String> to_string() const final override { return to_string({}); }
+    void serialize(Utf16StringBuilder&, SerializationParams const&) const;
+    Utf16String to_string(SerializationParams const&) const;
 
     static WebIDL::ExceptionOr<GC::Ref<CSSNumericValue>> parse(JS::VM&, String const& css_text);
 
diff --git a/Libraries/LibWeb/CSS/CSSPerspective.cpp b/Libraries/LibWeb/CSS/CSSPerspective.cpp
index 8004b88e5d..43adb2cfb7 100644
--- a/Libraries/LibWeb/CSS/CSSPerspective.cpp
+++ b/Libraries/LibWeb/CSS/CSSPerspective.cpp
@@ -87,22 +87,23 @@ void CSSPerspective::visit_edges(Visitor& visitor)
 WebIDL::ExceptionOr<Utf16String> CSSPerspective::to_string() const
 {
     // 1. Let s initially be "perspective(".
-    StringBuilder builder { StringBuilder::Mode::UTF16 };
-    builder.append("perspective("sv);
+    Utf16StringBuilder builder;
+    builder.append_ascii("perspective("sv);
 
     // 2. Serialize this’s length internal slot, with a minimum of 0px, and append it to s.
-    auto serialized_length = TRY(m_length.visit(
-        [](GC::Ref<CSSNumericValue> const& numeric_value) -> WebIDL::ExceptionOr<String> {
-            return numeric_value->to_string({ .minimum = 0 });
+    TRY(m_length.visit(
+        [&](GC::Ref<CSSNumericValue> const& numeric_value) -> WebIDL::ExceptionOr<void> {
+            numeric_value->serialize(builder, { .minimum = 0 });
+            return {};
         },
-        [](GC::Ref<CSSKeywordValue> const& keyword_value) -> WebIDL::ExceptionOr<String> {
-            return keyword_value->to_string();
+        [&](GC::Ref<CSSKeywordValue> const& keyword_value) -> WebIDL::ExceptionOr<void> {
+            builder.append(TRY(keyword_value->to_string()));
+            return {};
         }));
-    builder.append(serialized_length);
 
     // 3. Append ")" to s, and return s.
-    builder.append(")"sv);
-    return builder.to_utf16_string();
+    builder.append_ascii(')');
+    return builder.to_string();
 }
 
 WebIDL::ExceptionOr<GC::Ref<Geometry::DOMMatrix>> CSSPerspective::to_matrix() const
diff --git a/Libraries/LibWeb/CSS/CSSRotate.cpp b/Libraries/LibWeb/CSS/CSSRotate.cpp
index 5d49a98516..af2402ff14 100644
--- a/Libraries/LibWeb/CSS/CSSRotate.cpp
+++ b/Libraries/LibWeb/CSS/CSSRotate.cpp
@@ -98,49 +98,49 @@ void CSSRotate::visit_edges(Visitor& visitor)
 WebIDL::ExceptionOr<Utf16String> CSSRotate::to_string() const
 {
     // 1. Let s initially be the empty string.
-    StringBuilder builder { StringBuilder::Mode::UTF16 };
+    Utf16StringBuilder builder;
 
     // 2. If this’s is2D internal slot is false:
     if (!is_2d()) {
         // 1. Append "rotate3d(" to s.
-        builder.append("rotate3d("sv);
+        builder.append_ascii("rotate3d("sv);
 
         // 2. Serialize this’s x internal slot, and append it to s.
         m_x->serialize(builder, {});
 
         // 3. Append ", " to s.
-        builder.append(", "sv);
+        builder.append_ascii(", "sv);
 
         // 4. Serialize this’s y internal slot, and append it to s.
         m_y->serialize(builder, {});
 
         // 5. Append ", " to s.
-        builder.append(", "sv);
+        builder.append_ascii(", "sv);
 
         // 6. Serialize this’s z internal slot, and append it to s.
         m_z->serialize(builder, {});
 
         // 7. Append "," to s.
-        builder.append(", "sv);
+        builder.append_ascii(", "sv);
 
         // 8. Serialize this’s angle internal slot, and append it to s.
         m_angle->serialize(builder, {});
 
         // 9. Append ")" to s, and return s.
-        builder.append(")"sv);
-        return builder.to_utf16_string();
+        builder.append_ascii(')');
+        return builder.to_string();
     }
     // 2. Otherwise:
     else {
         // 1. Append "rotate(" to s.
-        builder.append("rotate("sv);
+        builder.append_ascii("rotate("sv);
 
         // 2. Serialize this’s angle internal slot, and append it to s.
         m_angle->serialize(builder, {});
 
         // 3. Append ")" to s, and return s.
-        builder.append(")"sv);
-        return builder.to_utf16_string();
+        builder.append_ascii(')');
+        return builder.to_string();
     }
 }
 
diff --git a/Libraries/LibWeb/CSS/CSSScale.cpp b/Libraries/LibWeb/CSS/CSSScale.cpp
index 88e83456f3..ee49356594 100644
--- a/Libraries/LibWeb/CSS/CSSScale.cpp
+++ b/Libraries/LibWeb/CSS/CSSScale.cpp
@@ -82,38 +82,38 @@ void CSSScale::visit_edges(Visitor& visitor)
 WebIDL::ExceptionOr<Utf16String> CSSScale::to_string() const
 {
     // 1. Let s initially be the empty string.
-    StringBuilder builder { StringBuilder::Mode::UTF16 };
+    Utf16StringBuilder builder;
 
     // 2. If this’s is2D internal slot is false:
     if (!is_2d()) {
 
         // 1. Append "scale3d(" to s.
-        builder.append("scale3d("sv);
+        builder.append_ascii("scale3d("sv);
 
         // 2. Serialize this’s x internal slot, and append it to s.
         m_x->serialize(builder, {});
 
         // 3. Append ", " to s.
-        builder.append(", "sv);
+        builder.append_ascii(", "sv);
 
         // 4. Serialize this’s y internal slot, and append it to s.
         m_y->serialize(builder, {});
 
         // 5. Append ", " to s.
-        builder.append(", "sv);
+        builder.append_ascii(", "sv);
 
         // 6. Serialize this’s z internal slot, and append it to s.
         m_z->serialize(builder, {});
 
         // 7. Append ")" to s, and return s.
-        builder.append(")"sv);
-        return builder.to_utf16_string();
+        builder.append_ascii(')');
+        return builder.to_string();
     }
 
     // 3. Otherwise:
     else {
         // 1. Append "scale(" to s.
-        builder.append("scale("sv);
+        builder.append_ascii("scale("sv);
 
         // 2. Serialize this’s x internal slot, and append it to s.
         m_x->serialize(builder, {});
@@ -123,14 +123,14 @@ WebIDL::ExceptionOr<Utf16String> CSSScale::to_string() const
         //         Upstream issue: https://github.com/w3c/css-houdini-drafts/issues/1161
 
         // 4. Otherwise, append ", " to s.
-        builder.append(", "sv);
+        builder.append_ascii(", "sv);
 
         // 5. Serialize this’s y internal slot, and append it to s.
         m_y->serialize(builder, {});
 
         // 6. Append ")" to s, and return s.
-        builder.append(")"sv);
-        return builder.to_utf16_string();
+        builder.append_ascii(')');
+        return builder.to_string();
     }
 }
 
diff --git a/Libraries/LibWeb/CSS/CSSSkew.cpp b/Libraries/LibWeb/CSS/CSSSkew.cpp
index 5004057fe8..ab5f4380ba 100644
--- a/Libraries/LibWeb/CSS/CSSSkew.cpp
+++ b/Libraries/LibWeb/CSS/CSSSkew.cpp
@@ -65,27 +65,27 @@ void CSSSkew::visit_edges(Visitor& visitor)
 WebIDL::ExceptionOr<Utf16String> CSSSkew::to_string() const
 {
     // 1. Let s initially be "skew(".
-    StringBuilder builder { StringBuilder::Mode::UTF16 };
-    builder.append("skew("sv);
+    Utf16StringBuilder builder;
+    builder.append_ascii("skew("sv);
 
     // 2. Serialize this’s ax internal slot, and append it to s.
     m_ax->serialize(builder, {});
 
     // 3. If this’s ay internal slot is a CSSUnitValue with a value of 0, then append ")" to s and return s.
     if (auto* ay_unit_value = as_if<CSSUnitValue>(*m_ay); ay_unit_value && ay_unit_value->value() == 0) {
-        builder.append(")"sv);
-        return builder.to_utf16_string();
+        builder.append_ascii(')');
+        return builder.to_string();
     }
 
     // 4. Otherwise, append ", " to s.
-    builder.append(", "sv);
+    builder.append_ascii(", "sv);
 
     // 5. Serialize this’s ay internal slot, and append it to s.
     m_ay->serialize(builder, {});
 
     // 6. Append ")" to s, and return s.
-    builder.append(")"sv);
-    return builder.to_utf16_string();
+    builder.append_ascii(')');
+    return builder.to_string();
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#dom-csstransformcomponent-tomatrix
diff --git a/Libraries/LibWeb/CSS/CSSSkewX.cpp b/Libraries/LibWeb/CSS/CSSSkewX.cpp
index 9bbd9edaf2..2aeb05a6b0 100644
--- a/Libraries/LibWeb/CSS/CSSSkewX.cpp
+++ b/Libraries/LibWeb/CSS/CSSSkewX.cpp
@@ -60,15 +60,15 @@ void CSSSkewX::visit_edges(Visitor& visitor)
 WebIDL::ExceptionOr<Utf16String> CSSSkewX::to_string() const
 {
     // 1. Let s initially be "skewX(".
-    StringBuilder builder { StringBuilder::Mode::UTF16 };
-    builder.append("skewX("sv);
+    Utf16StringBuilder builder;
+    builder.append_ascii("skewX("sv);
 
     // 2. Serialize this’s ax internal slot, and append it to s.
     m_ax->serialize(builder, {});
 
     // 3. Append ")" to s, and return s.
-    builder.append(")"sv);
-    return builder.to_utf16_string();
+    builder.append_ascii(')');
+    return builder.to_string();
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#dom-csstransformcomponent-tomatrix
diff --git a/Libraries/LibWeb/CSS/CSSSkewY.cpp b/Libraries/LibWeb/CSS/CSSSkewY.cpp
index 094247a324..0d89a7b7fb 100644
--- a/Libraries/LibWeb/CSS/CSSSkewY.cpp
+++ b/Libraries/LibWeb/CSS/CSSSkewY.cpp
@@ -60,15 +60,15 @@ void CSSSkewY::visit_edges(Visitor& visitor)
 WebIDL::ExceptionOr<Utf16String> CSSSkewY::to_string() const
 {
     // 1. Let s initially be "skewY(".
-    StringBuilder builder { StringBuilder::Mode::UTF16 };
-    builder.append("skewY("sv);
+    Utf16StringBuilder builder;
+    builder.append_ascii("skewY("sv);
 
     // 2. Serialize this’s ay internal slot, and append it to s.
     m_ay->serialize(builder, {});
 
     // 3. Append ")" to s, and return s.
-    builder.append(")"sv);
-    return builder.to_utf16_string();
+    builder.append_ascii(')');
+    return builder.to_string();
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#dom-csstransformcomponent-tomatrix
diff --git a/Libraries/LibWeb/CSS/CSSStyleValue.cpp b/Libraries/LibWeb/CSS/CSSStyleValue.cpp
index 283f525f37..3c72863719 100644
--- a/Libraries/LibWeb/CSS/CSSStyleValue.cpp
+++ b/Libraries/LibWeb/CSS/CSSStyleValue.cpp
@@ -104,7 +104,7 @@ WebIDL::ExceptionOr<Variant<GC::Ref<CSSStyleValue>, GC::RootVector<GC::Ref<CSSSt
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#stylevalue-serialization
-WebIDL::ExceptionOr<String> CSSStyleValue::to_string() const
+WebIDL::ExceptionOr<Utf16String> CSSStyleValue::to_string() const
 {
     // FIXME: if the value was constructed from a USVString
     // NB: Basically, if this was constructed with "parse a CSSStyleValue", regardless of what CSSStyleValue type it is now.
@@ -119,11 +119,11 @@ WebIDL::ExceptionOr<String> CSSStyleValue::to_string() const
     // FIXME: otherwise, if the value was extracted from the CSSOM
     // NB: For CSSStyleValue itself, we use the source value we were created from.
     if (m_source_value)
-        return m_source_value->to_string(SerializationMode::Normal);
+        return Utf16String::from_utf8_without_validation(m_source_value->to_string(SerializationMode::Normal));
     {
         // the serialization is specified in §6.7 Serialization from CSSOM Values below.
     }
-    return String {};
+    return Utf16String::from_utf8_without_validation(""sv);
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#create-an-internal-representation
diff --git a/Libraries/LibWeb/CSS/CSSStyleValue.h b/Libraries/LibWeb/CSS/CSSStyleValue.h
index cd0adada1b..9d606caf96 100644
--- a/Libraries/LibWeb/CSS/CSSStyleValue.h
+++ b/Libraries/LibWeb/CSS/CSSStyleValue.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/Utf16FlyString.h>
+#include <AK/Utf16String.h>
 #include <LibWeb/Bindings/PlatformObject.h>
 
 namespace Web::CSS {
@@ -35,7 +36,7 @@ public:
     };
     static WebIDL::ExceptionOr<Variant<GC::Ref<CSSStyleValue>, GC::RootVector<GC::Ref<CSSStyleValue>>>> parse_a_css_style_value(JS::VM&, Utf16FlyString property, String css_text, ParseMultiple);
 
-    virtual WebIDL::ExceptionOr<String> to_string() const;
+    virtual WebIDL::ExceptionOr<Utf16String> to_string() const;
 
     // FIXME: Temporary hack. Really we want to pass something like a CalculationContext with the valid types and ranges.
     enum class PerformTypeCheck : u8 {
diff --git a/Libraries/LibWeb/CSS/CSSTransformValue.cpp b/Libraries/LibWeb/CSS/CSSTransformValue.cpp
index 069da23187..298b9e193d 100644
--- a/Libraries/LibWeb/CSS/CSSTransformValue.cpp
+++ b/Libraries/LibWeb/CSS/CSSTransformValue.cpp
@@ -5,6 +5,7 @@
  */
 
 #include "CSSTransformValue.h"
+#include <AK/Utf16StringBuilder.h>
 #include <LibWeb/Bindings/CSSTransformValue.h>
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/CSS/CSSTransformComponent.h>
@@ -141,19 +142,19 @@ WebIDL::ExceptionOr<GC::Ref<Geometry::DOMMatrix>> CSSTransformValue::to_matrix()
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-csstransformvalue
-WebIDL::ExceptionOr<String> CSSTransformValue::to_string() const
+WebIDL::ExceptionOr<Utf16String> CSSTransformValue::to_string() const
 {
     // 1. Return the result of serializing each item in this’s values to iterate over, then concatenating them
     //    separated by " ".
-    StringBuilder builder;
+    Utf16StringBuilder builder;
     bool first = true;
     for (auto const& transform : m_transforms) {
         if (!first)
-            builder.append(" "sv);
+            builder.append_ascii(' ');
         first = false;
         builder.append(TRY(transform->to_string()));
     }
-    return builder.to_string_without_validation();
+    return builder.to_string();
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#create-an-internal-representation
diff --git a/Libraries/LibWeb/CSS/CSSTransformValue.h b/Libraries/LibWeb/CSS/CSSTransformValue.h
index a830defd0b..3c2567f5f2 100644
--- a/Libraries/LibWeb/CSS/CSSTransformValue.h
+++ b/Libraries/LibWeb/CSS/CSSTransformValue.h
@@ -30,7 +30,7 @@ public:
     bool is_2d() const;
     WebIDL::ExceptionOr<GC::Ref<Geometry::DOMMatrix>> to_matrix() const;
 
-    virtual WebIDL::ExceptionOr<String> to_string() const override;
+    virtual WebIDL::ExceptionOr<Utf16String> to_string() const override;
 
     virtual WebIDL::ExceptionOr<NonnullRefPtr<StyleValue const>> create_an_internal_representation(PropertyNameAndID const&, PerformTypeCheck) const override;
 
diff --git a/Libraries/LibWeb/CSS/CSSTranslate.cpp b/Libraries/LibWeb/CSS/CSSTranslate.cpp
index 53f1694573..2fee033d86 100644
--- a/Libraries/LibWeb/CSS/CSSTranslate.cpp
+++ b/Libraries/LibWeb/CSS/CSSTranslate.cpp
@@ -81,49 +81,49 @@ void CSSTranslate::visit_edges(Visitor& visitor)
 WebIDL::ExceptionOr<Utf16String> CSSTranslate::to_string() const
 {
     // 1. Let s initially be the empty string.
-    StringBuilder builder { StringBuilder::Mode::UTF16 };
+    Utf16StringBuilder builder;
 
     // 2. If this’s is2D internal slot is false:
     if (!is_2d()) {
         // 1. Append "translate3d(" to s.
-        builder.append("translate3d("sv);
+        builder.append_ascii("translate3d("sv);
 
         // 2. Serialize this’s x internal slot, and append it to s.
         m_x->serialize(builder, {});
 
         // 3. Append ", " to s.
-        builder.append(", "sv);
+        builder.append_ascii(", "sv);
 
         // 4. Serialize this’s y internal slot, and append it to s.
         m_y->serialize(builder, {});
 
         // 5. Append ", " to s.
-        builder.append(", "sv);
+        builder.append_ascii(", "sv);
 
         // 6. Serialize this’s z internal slot, and append it to s.
         m_z->serialize(builder, {});
 
         // 7. Append ")" to s, and return s.
-        builder.append(")"sv);
-        return builder.to_utf16_string();
+        builder.append_ascii(')');
+        return builder.to_string();
     }
     // 3. Otherwise:
     else {
         // 1. Append "translate(" to s.
-        builder.append("translate("sv);
+        builder.append_ascii("translate("sv);
 
         // 2. Serialize this’s x internal slot, and append it to s.
         m_x->serialize(builder, {});
 
         // 3. Append ", " to s.
-        builder.append(", "sv);
+        builder.append_ascii(", "sv);
 
         // 4. Serialize this’s y internal slot, and append it to s.
         m_y->serialize(builder, {});
 
         // 5. Append ")" to s, and return s.
-        builder.append(")"sv);
-        return builder.to_utf16_string();
+        builder.append_ascii(')');
+        return builder.to_string();
     }
 }
 
diff --git a/Libraries/LibWeb/CSS/CSSUnitValue.cpp b/Libraries/LibWeb/CSS/CSSUnitValue.cpp
index ab56e875a2..49e788fcbe 100644
--- a/Libraries/LibWeb/CSS/CSSUnitValue.cpp
+++ b/Libraries/LibWeb/CSS/CSSUnitValue.cpp
@@ -92,7 +92,7 @@ void CSSUnitValue::set_value(double value)
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssunitvalue
-void CSSUnitValue::serialize_unit_value(StringBuilder& builder, Optional<double> minimum, Optional<double> maximum) const
+void CSSUnitValue::serialize_unit_value(Utf16StringBuilder& builder, Optional<double> minimum, Optional<double> maximum) const
 {
     // To serialize a CSSUnitValue this, with optional arguments minimum, a numeric value, and maximum, a numeric value:
 
@@ -107,7 +107,7 @@ void CSSUnitValue::serialize_unit_value(StringBuilder& builder, Optional<double>
         || (maximum.has_value() && m_value > maximum.value());
 
     if (needs_calc_wrapper)
-        builder.append("calc("sv);
+        builder.append_ascii("calc("sv);
 
     // 2. Set s to the result of serializing a <number> from value, per CSSOM §6.7.2 Serializing CSS Values.
     serialize_a_number(builder, m_value);
@@ -120,16 +120,16 @@ void CSSUnitValue::serialize_unit_value(StringBuilder& builder, Optional<double>
     // -> "percent"
     else if (m_unit == "percent"_fly_string) {
         // Append "%" to s.
-        builder.append('%');
+        builder.append_ascii('%');
     }
     // -> anything else
     else {
         // Append unit to s.
-        builder.append(m_unit.to_ascii_lowercase());
+        builder.append_ascii(m_unit.to_ascii_lowercase().bytes_as_string_view());
     }
 
     if (needs_calc_wrapper)
-        builder.append(')');
+        builder.append_ascii(')');
 
     // 5. Return s.
 }
diff --git a/Libraries/LibWeb/CSS/CSSUnitValue.h b/Libraries/LibWeb/CSS/CSSUnitValue.h
index 1ceb0cae71..5c64763117 100644
--- a/Libraries/LibWeb/CSS/CSSUnitValue.h
+++ b/Libraries/LibWeb/CSS/CSSUnitValue.h
@@ -28,7 +28,7 @@ public:
 
     FlyString const& unit() const { return m_unit; }
 
-    void serialize_unit_value(StringBuilder&, Optional<double> minimum, Optional<double> maximum) const;
+    void serialize_unit_value(Utf16StringBuilder&, Optional<double> minimum, Optional<double> maximum) const;
 
     GC::Ptr<CSSUnitValue> converted_to_unit(FlyString const& unit) const;
 
diff --git a/Libraries/LibWeb/CSS/CSSUnparsedValue.cpp b/Libraries/LibWeb/CSS/CSSUnparsedValue.cpp
index 628e87e8c2..23b0702041 100644
--- a/Libraries/LibWeb/CSS/CSSUnparsedValue.cpp
+++ b/Libraries/LibWeb/CSS/CSSUnparsedValue.cpp
@@ -5,6 +5,7 @@
  */
 
 #include "CSSUnparsedValue.h"
+#include <AK/Utf16StringBuilder.h>
 #include <LibWeb/Bindings/CSSUnparsedValue.h>
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/CSS/CSSVariableReferenceValue.h>
@@ -131,17 +132,17 @@ bool CSSUnparsedValue::contains_unparsed_value(CSSUnparsedValue const& needle) c
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssunparsedvalue
-WebIDL::ExceptionOr<String> CSSUnparsedValue::to_string() const
+WebIDL::ExceptionOr<Utf16String> CSSUnparsedValue::to_string() const
 {
     // AD-HOC: It's possible for one of the m_tokens to contain this in its fallback slot, or a similar situation with
     //         more levels of nesting. To avoid crashing, do a scan for that first and return the empty string.
     // Spec issue: https://github.com/w3c/css-houdini-drafts/issues/1158
     if (contains_unparsed_value(*this))
-        return ""_string;
+        return Utf16String::from_utf8_without_validation(""sv);
 
     // To serialize a CSSUnparsedValue this:
     // 1. Let s initially be the empty string.
-    StringBuilder s;
+    Utf16StringBuilder s;
 
     // 2. For each item in this’s [[tokens]] internal slot:
     for (auto const& item : m_tokens) {
@@ -150,7 +151,7 @@ WebIDL::ExceptionOr<String> CSSUnparsedValue::to_string() const
         TRY(item.visit(
             // 1. If item is a USVString, append it to s.
             [&](String const& string) -> WebIDL::ExceptionOr<void> {
-                s.append(string);
+                s.append(Utf16String::from_utf8_without_validation(string));
                 return {};
             },
             // 2. Otherwise, item is a CSSVariableReferenceValue. Serialize it, then append the result to s.
@@ -161,7 +162,7 @@ WebIDL::ExceptionOr<String> CSSUnparsedValue::to_string() const
     }
 
     // 3. Return s.
-    return s.to_string_without_validation();
+    return s.to_string();
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#create-an-internal-representation
@@ -181,7 +182,8 @@ WebIDL::ExceptionOr<NonnullRefPtr<StyleValue const>> CSSUnparsedValue::create_an
 
     // NB: CSSUnparsedValue stores a list of strings, each of which may contain any number of tokens. So the simplest
     //     way to convert it to ComponentValues is to serialize and then parse it.
-    auto string = TRY(to_string());
+    auto utf16_string = TRY(to_string());
+    auto string = MUST(utf16_string.utf16_view().to_utf8());
     auto parser = Parser::Parser::create(Parser::ParsingParams {}, string);
     auto component_values = parser.parse_as_list_of_component_values();
 
diff --git a/Libraries/LibWeb/CSS/CSSUnparsedValue.h b/Libraries/LibWeb/CSS/CSSUnparsedValue.h
index 7127575190..e72560d7d7 100644
--- a/Libraries/LibWeb/CSS/CSSUnparsedValue.h
+++ b/Libraries/LibWeb/CSS/CSSUnparsedValue.h
@@ -29,7 +29,7 @@ public:
     virtual WebIDL::ExceptionOr<void> set_value_of_existing_indexed_property(u32, JS::Value) override;
     virtual WebIDL::ExceptionOr<void> set_value_of_new_indexed_property(u32, JS::Value) override;
 
-    virtual WebIDL::ExceptionOr<String> to_string() const override;
+    virtual WebIDL::ExceptionOr<Utf16String> to_string() const override;
     virtual WebIDL::ExceptionOr<NonnullRefPtr<StyleValue const>> create_an_internal_representation(PropertyNameAndID const&, PerformTypeCheck) const override;
 
 private:
diff --git a/Libraries/LibWeb/CSS/CSSVariableReferenceValue.cpp b/Libraries/LibWeb/CSS/CSSVariableReferenceValue.cpp
index ac8ec9efba..dea540fb7c 100644
--- a/Libraries/LibWeb/CSS/CSSVariableReferenceValue.cpp
+++ b/Libraries/LibWeb/CSS/CSSVariableReferenceValue.cpp
@@ -5,6 +5,7 @@
  */
 
 #include "CSSVariableReferenceValue.h"
+#include <AK/Utf16StringBuilder.h>
 #include <LibWeb/Bindings/CSSVariableReferenceValue.h>
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/CSS/CSSUnparsedValue.h>
@@ -89,26 +90,26 @@ WebIDL::ExceptionOr<void> CSSVariableReferenceValue::set_fallback(GC::Ptr<CSSUnp
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssvariablereferencevalue
-WebIDL::ExceptionOr<String> CSSVariableReferenceValue::to_string() const
+WebIDL::ExceptionOr<Utf16String> CSSVariableReferenceValue::to_string() const
 {
     // To serialize a CSSVariableReferenceValue this:
     // 1. Let s initially be "var(".
-    StringBuilder s;
-    s.append("var("sv);
+    Utf16StringBuilder s;
+    s.append_ascii("var("sv);
 
     // 2. Append this’s variable internal slot to s.
-    s.append(m_variable);
+    s.append(Utf16String::from_utf8_without_validation(m_variable));
 
     // 3. If this’s fallback internal slot is not null, append ", " to s, then serialize the fallback internal slot and append it to s.
     if (m_fallback) {
         // AD-HOC: Tested behaviour requires we append "," without the space. https://github.com/w3c/css-houdini-drafts/issues/1148
-        s.append(","sv);
+        s.append_ascii(',');
         s.append(TRY(m_fallback->to_string()));
     }
 
     // 4. Append ")" to s and return s.
-    s.append(")"sv);
-    return s.to_string_without_validation();
+    s.append_ascii(')');
+    return s.to_string();
 }
 
 }
diff --git a/Libraries/LibWeb/CSS/CSSVariableReferenceValue.h b/Libraries/LibWeb/CSS/CSSVariableReferenceValue.h
index 30f52c4b57..25bf326fef 100644
--- a/Libraries/LibWeb/CSS/CSSVariableReferenceValue.h
+++ b/Libraries/LibWeb/CSS/CSSVariableReferenceValue.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/FlyString.h>
+#include <AK/Utf16String.h>
 #include <LibWeb/Bindings/PlatformObject.h>
 
 namespace Web::CSS {
@@ -27,7 +28,7 @@ public:
     GC::Ptr<CSSUnparsedValue> fallback() const;
     WebIDL::ExceptionOr<void> set_fallback(GC::Ptr<CSSUnparsedValue>);
 
-    WebIDL::ExceptionOr<String> to_string() const;
+    WebIDL::ExceptionOr<Utf16String> to_string() const;
 
 private:
     CSSVariableReferenceValue(JS::Realm&, FlyString variable, GC::Ptr<CSSUnparsedValue> fallback);
diff --git a/Libraries/LibWeb/CSS/Serialize.cpp b/Libraries/LibWeb/CSS/Serialize.cpp
index 0651313de3..eff0c79dd0 100644
--- a/Libraries/LibWeb/CSS/Serialize.cpp
+++ b/Libraries/LibWeb/CSS/Serialize.cpp
@@ -149,6 +149,24 @@ void serialize_a_number(StringBuilder& builder, double value)
     builder.appendff("{:.6}", value);
 }
 
+void serialize_a_number(Utf16StringBuilder& builder, double value)
+{
+    // -> <number>
+    //    A base-ten number using digits 0-9 (U+0030 to U+0039) in the shortest form possible, using "." to separate
+    //    decimals (if any), rounding the value if necessary to not produce more than 6 decimals, preceded by "-"
+    //    (U+002D) if it is negative.
+    // NOTE: scientific notation is not used.
+
+    // AD-HOC: If the number is small enough that it would not print any digits when rounded, serialize it as 0.
+    if (AK::abs(value) < 0.0000005) {
+        builder.append_ascii('0');
+        return;
+    }
+
+    // FIXME: Prevent scientific notation for large values.
+    builder.appendff("{:.6}", value);
+}
+
 String serialize_an_identifier(StringView ident)
 {
     StringBuilder builder;
diff --git a/Libraries/LibWeb/CSS/Serialize.h b/Libraries/LibWeb/CSS/Serialize.h
index 099119b883..5917dcc86f 100644
--- a/Libraries/LibWeb/CSS/Serialize.h
+++ b/Libraries/LibWeb/CSS/Serialize.h
@@ -9,6 +9,7 @@
 #include <AK/String.h>
 #include <AK/StringBuilder.h>
 #include <AK/StringView.h>
+#include <AK/Utf16StringBuilder.h>
 #include <AK/Vector.h>
 #include <LibGfx/Color.h>
 #include <LibGfx/Font/UnicodeRange.h>
@@ -25,6 +26,7 @@ void serialize_a_string(StringBuilder&, StringView string);
 WEB_API void serialize_a_url(StringBuilder&, StringView url);
 void serialize_unicode_ranges(StringBuilder&, Vector<Gfx::UnicodeRange> const& unicode_ranges);
 WEB_API void serialize_a_number(StringBuilder&, double value);
+WEB_API void serialize_a_number(Utf16StringBuilder&, double value);
 
 String serialize_an_identifier(StringView ident);
 String serialize_a_string(StringView string);