LibWeb: Add default user agent style sheet for Fullscreen API

Matching the fullscreen pseudo class is currently a stub that will be
implemented in a future commit.

Libraries/LibWeb/CSS/Default.css

@@ -914,6 +914,32 @@ button, meter, progress, select {
     animation-play-state: inherit;
 }
 
+/* Fullscreen API defaults https://fullscreen.spec.whatwg.org/#user-agent-level-style-sheet-defaults */
+*|*:not(:root):fullscreen {
+  position:fixed !important;
+  inset:0 !important;
+  margin:0 !important;
+  box-sizing:border-box !important;
+  min-width:0 !important;
+  max-width:none !important;
+  min-height:0 !important;
+  max-height:none !important;
+  width:100% !important;
+  height:100% !important;
+  transform:none !important;
+  /* intentionally not !important */
+  object-fit:contain;
+}
+
+iframe:fullscreen {
+  border:none !important;
+  padding:0 !important;
+}
+
+*|*:not(:root):fullscreen::backdrop {
+  background:black;
+}
+
 /* Default cross-fade transition */
 @keyframes -ua-view-transition-fade-out {
     to { opacity: 0; }

Libraries/LibWeb/CSS/PseudoClasses.json

@@ -53,6 +53,9 @@
   "focus-within": {
     "argument": ""
   },
+  "fullscreen": {
+    "argument": ""
+  },
   "has": {
     "argument": "<relative-selector-list>"
   },

Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -9,6 +9,7 @@
 #include <LibWeb/CSS/ComputedProperties.h>
 #include <LibWeb/CSS/Keyword.h>
 #include <LibWeb/CSS/Parser/Parser.h>
+#include <LibWeb/CSS/PseudoClass.h>
 #include <LibWeb/CSS/SelectorEngine.h>
 #include <LibWeb/DOM/Attr.h>
 #include <LibWeb/DOM/Document.h>
@@ -557,6 +558,10 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
         auto focused_area = element.document().focused_area();
         return focused_area && element.is_inclusive_ancestor_of(*focused_area);
     }
+    case CSS::PseudoClass::Fullscreen: {
+        // FIXME: Add fullscreen support
+        return false;
+    }
     case CSS::PseudoClass::FirstChild:
         if (context.collect_per_element_selector_involvement_metadata) {
             const_cast<DOM::Element&>(element).set_affected_by_sibling_position_or_count_pseudo_class(true);

Libraries/LibWeb/DOM/StyleInvalidationReason.h

@@ -25,6 +25,7 @@ namespace Web::DOM {
     X(EditingDeletion)                              \
     X(ElementAttributeChange)                       \
     X(ElementSetShadowRoot)                         \
+    X(Fullscreen)                                   \
     X(HTMLDialogElementSetIsModal)                  \
     X(HTMLDetailsOrDialogOpenAttributeChange)       \
     X(HTMLHyperlinkElementHrefChange)               \