LibWeb+LibWebView+WebContent: Add support for meta theme-color

Userland/Libraries/LibWeb/HTML/HTMLMetaElement.cpp

@@ -6,8 +6,14 @@
  */
 
 #include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/CSS/Parser/Parser.h>
+#include <LibWeb/CSS/Parser/ParsingContext.h>
+#include <LibWeb/CSS/PropertyID.h>
+#include <LibWeb/CSS/StyleValues/ColorStyleValue.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/HTML/HTMLMetaElement.h>
+#include <LibWeb/Infra/CharacterTypes.h>
+#include <LibWeb/Page/Page.h>
 
 namespace Web::HTML {
 
@@ -41,6 +47,43 @@ void HTMLMetaElement::inserted()
 {
     Base::inserted();
 
+    // https://html.spec.whatwg.org/multipage/semantics.html#meta-theme-color
+    // 1. To obtain a page's theme color, user agents must run the following steps:
+    //     * The element is in a document tree
+    //     * The element has a name attribute, whose value is an ASCII case-insensitive match for theme-color
+    //     * The element has a content attribute
+    auto name = attribute(AttributeNames::name);
+    auto content = attribute(AttributeNames::content);
+    if (name.has_value() && name->bytes_as_string_view().equals_ignoring_ascii_case("theme-color"sv) && content.has_value()) {
+        auto* page = document().page();
+        if (!page)
+            return;
+        auto context = CSS::Parser::ParsingContext { document() };
+
+        // 2. For each element in candidate elements:
+
+        // 1. If element has a media attribute and the value of element's media attribute does not match the environment, then continue.
+        auto media = attribute(AttributeNames::media);
+        if (media.has_value()) {
+            auto query = parse_media_query(context, media.value());
+            if (!query->evaluate(document().window()))
+                return;
+        }
+
+        // 2. Let value be the result of stripping leading and trailing ASCII whitespace from the value of element's content attribute.
+        auto value = content->bytes_as_string_view().trim(Infra::ASCII_WHITESPACE);
+
+        // 3. Let color be the result of parsing value.
+        auto css_value = parse_css_value(context, value, CSS::PropertyID::Color);
+        if (css_value.is_null() || !css_value->is_color())
+            return;
+        auto color = css_value->as_color().color();
+
+        // 4. If color is not failure, then return color.
+        page->client().page_did_change_theme_color(color);
+        return;
+    }
+
     // https://html.spec.whatwg.org/multipage/semantics.html#pragma-directives
     // When a meta element is inserted into the document, if its http-equiv attribute is present and represents one of
     // the above states, then the user agent must run the algorithm appropriate for that state, as described in the