diff --git a/Libraries/LibWeb/CSS/Parser/PropertyParsing.cpp b/Libraries/LibWeb/CSS/Parser/PropertyParsing.cpp
index e12487fd55..424d24be45 100644
--- a/Libraries/LibWeb/CSS/Parser/PropertyParsing.cpp
+++ b/Libraries/LibWeb/CSS/Parser/PropertyParsing.cpp
@@ -2242,6 +2242,11 @@ RefPtr<StyleValue const> Parser::parse_content_value(TokenStream<ComponentValue>
                 return nullptr;
 
             if (in_alt_text) {
+                // https://drafts.csswg.org/css-content-3/#content-property
+                // / [ <string> | <counter> | <attr()> ]+
+                // NB: <attr()> is handled as an arbitrary substitution function and does not reach this code path.
+                if (!style_value->is_string() && !style_value->is_counter())
+                    return nullptr;
                 alt_text_values.append(style_value.release_nonnull());
             } else {
                 content_values.append(style_value.release_nonnull());
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-content/parsing/content-invalid.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-content/parsing/content-invalid.txt
index befb30f7a3..7172124d28 100644
--- a/Tests/LibWeb/Text/expected/wpt-import/css/css-content/parsing/content-invalid.txt
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-content/parsing/content-invalid.txt
@@ -2,75 +2,75 @@ Harness status: OK
 
 Found 70 tests
 
-4 Pass
-66 Fail
+68 Pass
+2 Fail
 Fail	e.style['content'] = "attr()" should not set the property value
 Fail	e.style['content'] = "attr() / \"alt text\"" should not set the property value
 Pass	e.style['content'] = "counters(counter-name)" should not set the property value
 Pass	e.style['content'] = "counters(counter-name) / \"alt text\"" should not set the property value
 Pass	e.style['content'] = "counter()" should not set the property value
 Pass	e.style['content'] = "counter() / \"alt text\"" should not set the property value
-Fail	e.style['content'] = "open-quote / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "open-quote / no-open-quote" should not set the property value
-Fail	e.style['content'] = "open-quote / no-close-quote" should not set the property value
-Fail	e.style['content'] = "open-quote / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "close-quote / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "close-quote / no-open-quote" should not set the property value
-Fail	e.style['content'] = "close-quote / no-close-quote" should not set the property value
-Fail	e.style['content'] = "close-quote / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "no-open-quote / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "no-open-quote / no-open-quote" should not set the property value
-Fail	e.style['content'] = "no-open-quote / no-close-quote" should not set the property value
-Fail	e.style['content'] = "no-open-quote / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "no-close-quote / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "no-close-quote / no-open-quote" should not set the property value
-Fail	e.style['content'] = "no-close-quote / no-close-quote" should not set the property value
-Fail	e.style['content'] = "no-close-quote / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "counter(counter-name) / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "counter(counter-name) / no-open-quote" should not set the property value
-Fail	e.style['content'] = "counter(counter-name) / no-close-quote" should not set the property value
-Fail	e.style['content'] = "counter(counter-name) / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "counter(counter-name, counter-style) / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "counter(counter-name, counter-style) / no-open-quote" should not set the property value
-Fail	e.style['content'] = "counter(counter-name, counter-style) / no-close-quote" should not set the property value
-Fail	e.style['content'] = "counter(counter-name, counter-style) / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "counters(counter-name, \".\") / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "counters(counter-name, \".\") / no-open-quote" should not set the property value
-Fail	e.style['content'] = "counters(counter-name, \".\") / no-close-quote" should not set the property value
-Fail	e.style['content'] = "counters(counter-name, \".\") / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "counters(counter-name, \".\", counter-style) / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "counters(counter-name, \".\", counter-style) / no-open-quote" should not set the property value
-Fail	e.style['content'] = "counters(counter-name, \".\", counter-style) / no-close-quote" should not set the property value
-Fail	e.style['content'] = "counters(counter-name, \".\", counter-style) / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "url(\"https://www.example.com/picture.svg\") / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "url(\"https://www.example.com/picture.svg\") / no-open-quote" should not set the property value
-Fail	e.style['content'] = "url(\"https://www.example.com/picture.svg\") / no-close-quote" should not set the property value
-Fail	e.style['content'] = "url(\"https://www.example.com/picture.svg\") / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "\"hello\" / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "\"hello\" / no-open-quote" should not set the property value
-Fail	e.style['content'] = "\"hello\" / no-close-quote" should not set the property value
-Fail	e.style['content'] = "\"hello\" / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "\"hello\" \"world\" / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "\"hello\" \"world\" / no-open-quote" should not set the property value
-Fail	e.style['content'] = "\"hello\" \"world\" / no-close-quote" should not set the property value
-Fail	e.style['content'] = "\"hello\" \"world\" / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "counter(counter-name) \"potato\" / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "counter(counter-name) \"potato\" / no-open-quote" should not set the property value
-Fail	e.style['content'] = "counter(counter-name) \"potato\" / no-close-quote" should not set the property value
-Fail	e.style['content'] = "counter(counter-name) \"potato\" / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "counters(counter-name, \".\") \"potato\" / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "counters(counter-name, \".\") \"potato\" / no-open-quote" should not set the property value
-Fail	e.style['content'] = "counters(counter-name, \".\") \"potato\" / no-close-quote" should not set the property value
-Fail	e.style['content'] = "counters(counter-name, \".\") \"potato\" / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "\"(\" counters(counter-name, \".\", counter-style) \")\" / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "\"(\" counters(counter-name, \".\", counter-style) \")\" / no-open-quote" should not set the property value
-Fail	e.style['content'] = "\"(\" counters(counter-name, \".\", counter-style) \")\" / no-close-quote" should not set the property value
-Fail	e.style['content'] = "\"(\" counters(counter-name, \".\", counter-style) \")\" / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "open-quote \"hello\" \"world\" close-quote / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "open-quote \"hello\" \"world\" close-quote / no-open-quote" should not set the property value
-Fail	e.style['content'] = "open-quote \"hello\" \"world\" close-quote / no-close-quote" should not set the property value
-Fail	e.style['content'] = "open-quote \"hello\" \"world\" close-quote / \"hi\" no-close-quote" should not set the property value
-Fail	e.style['content'] = "url(\"https://www.example.com/picture.svg\") \"hello\" / url(\"https://www.example.com/picture.svg\")" should not set the property value
-Fail	e.style['content'] = "url(\"https://www.example.com/picture.svg\") \"hello\" / no-open-quote" should not set the property value
-Fail	e.style['content'] = "url(\"https://www.example.com/picture.svg\") \"hello\" / no-close-quote" should not set the property value
-Fail	e.style['content'] = "url(\"https://www.example.com/picture.svg\") \"hello\" / \"hi\" no-close-quote" should not set the property value
\ No newline at end of file
+Pass	e.style['content'] = "open-quote / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "open-quote / no-open-quote" should not set the property value
+Pass	e.style['content'] = "open-quote / no-close-quote" should not set the property value
+Pass	e.style['content'] = "open-quote / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "close-quote / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "close-quote / no-open-quote" should not set the property value
+Pass	e.style['content'] = "close-quote / no-close-quote" should not set the property value
+Pass	e.style['content'] = "close-quote / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "no-open-quote / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "no-open-quote / no-open-quote" should not set the property value
+Pass	e.style['content'] = "no-open-quote / no-close-quote" should not set the property value
+Pass	e.style['content'] = "no-open-quote / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "no-close-quote / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "no-close-quote / no-open-quote" should not set the property value
+Pass	e.style['content'] = "no-close-quote / no-close-quote" should not set the property value
+Pass	e.style['content'] = "no-close-quote / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "counter(counter-name) / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "counter(counter-name) / no-open-quote" should not set the property value
+Pass	e.style['content'] = "counter(counter-name) / no-close-quote" should not set the property value
+Pass	e.style['content'] = "counter(counter-name) / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "counter(counter-name, counter-style) / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "counter(counter-name, counter-style) / no-open-quote" should not set the property value
+Pass	e.style['content'] = "counter(counter-name, counter-style) / no-close-quote" should not set the property value
+Pass	e.style['content'] = "counter(counter-name, counter-style) / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "counters(counter-name, \".\") / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "counters(counter-name, \".\") / no-open-quote" should not set the property value
+Pass	e.style['content'] = "counters(counter-name, \".\") / no-close-quote" should not set the property value
+Pass	e.style['content'] = "counters(counter-name, \".\") / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "counters(counter-name, \".\", counter-style) / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "counters(counter-name, \".\", counter-style) / no-open-quote" should not set the property value
+Pass	e.style['content'] = "counters(counter-name, \".\", counter-style) / no-close-quote" should not set the property value
+Pass	e.style['content'] = "counters(counter-name, \".\", counter-style) / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "url(\"https://www.example.com/picture.svg\") / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "url(\"https://www.example.com/picture.svg\") / no-open-quote" should not set the property value
+Pass	e.style['content'] = "url(\"https://www.example.com/picture.svg\") / no-close-quote" should not set the property value
+Pass	e.style['content'] = "url(\"https://www.example.com/picture.svg\") / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "\"hello\" / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "\"hello\" / no-open-quote" should not set the property value
+Pass	e.style['content'] = "\"hello\" / no-close-quote" should not set the property value
+Pass	e.style['content'] = "\"hello\" / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "\"hello\" \"world\" / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "\"hello\" \"world\" / no-open-quote" should not set the property value
+Pass	e.style['content'] = "\"hello\" \"world\" / no-close-quote" should not set the property value
+Pass	e.style['content'] = "\"hello\" \"world\" / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "counter(counter-name) \"potato\" / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "counter(counter-name) \"potato\" / no-open-quote" should not set the property value
+Pass	e.style['content'] = "counter(counter-name) \"potato\" / no-close-quote" should not set the property value
+Pass	e.style['content'] = "counter(counter-name) \"potato\" / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "counters(counter-name, \".\") \"potato\" / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "counters(counter-name, \".\") \"potato\" / no-open-quote" should not set the property value
+Pass	e.style['content'] = "counters(counter-name, \".\") \"potato\" / no-close-quote" should not set the property value
+Pass	e.style['content'] = "counters(counter-name, \".\") \"potato\" / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "\"(\" counters(counter-name, \".\", counter-style) \")\" / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "\"(\" counters(counter-name, \".\", counter-style) \")\" / no-open-quote" should not set the property value
+Pass	e.style['content'] = "\"(\" counters(counter-name, \".\", counter-style) \")\" / no-close-quote" should not set the property value
+Pass	e.style['content'] = "\"(\" counters(counter-name, \".\", counter-style) \")\" / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "open-quote \"hello\" \"world\" close-quote / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "open-quote \"hello\" \"world\" close-quote / no-open-quote" should not set the property value
+Pass	e.style['content'] = "open-quote \"hello\" \"world\" close-quote / no-close-quote" should not set the property value
+Pass	e.style['content'] = "open-quote \"hello\" \"world\" close-quote / \"hi\" no-close-quote" should not set the property value
+Pass	e.style['content'] = "url(\"https://www.example.com/picture.svg\") \"hello\" / url(\"https://www.example.com/picture.svg\")" should not set the property value
+Pass	e.style['content'] = "url(\"https://www.example.com/picture.svg\") \"hello\" / no-open-quote" should not set the property value
+Pass	e.style['content'] = "url(\"https://www.example.com/picture.svg\") \"hello\" / no-close-quote" should not set the property value
+Pass	e.style['content'] = "url(\"https://www.example.com/picture.svg\") \"hello\" / \"hi\" no-close-quote" should not set the property value
\ No newline at end of file