diff --git a/Libraries/LibWeb/CSS/Parser/Helpers.cpp b/Libraries/LibWeb/CSS/Parser/Helpers.cpp
index 52f7cb22d73..c24ecf70bba 100644
--- a/Libraries/LibWeb/CSS/Parser/Helpers.cpp
+++ b/Libraries/LibWeb/CSS/Parser/Helpers.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2022, Andreas Kling <andreas@ladybird.org>
+ * Copyright (c) 2018-2025, Andreas Kling <andreas@ladybird.org>
  * Copyright (c) 2020-2023, the SerenityOS developers.
  * Copyright (c) 2021-2024, Sam Atkins <atkinssj@serenityos.org>
  * Copyright (c) 2021, Tobias Christiansen <tobyase@serenityos.org>
@@ -8,13 +8,40 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibWeb/Bindings/MainThreadVM.h>
+#include <LibWeb/Bindings/PrincipalHostDefined.h>
 #include <LibWeb/CSS/CSSMediaRule.h>
 #include <LibWeb/CSS/CSSRuleList.h>
 #include <LibWeb/CSS/CSSStyleSheet.h>
 #include <LibWeb/CSS/Parser/Parser.h>
+#include <LibWeb/HTML/Window.h>
 
 namespace Web {
 
+GC::Ref<JS::Realm> internal_css_realm()
+{
+    static GC::Root<JS::Realm> realm;
+    static GC::Root<HTML::Window> window;
+    static OwnPtr<JS::ExecutionContext> execution_context;
+    if (!realm) {
+        execution_context = Bindings::create_a_new_javascript_realm(
+            Bindings::main_thread_vm(),
+            [&](JS::Realm& realm) -> JS::Object* {
+                window = HTML::Window::create(realm);
+                return window;
+            },
+            [&](JS::Realm&) -> JS::Object* {
+                return window;
+            });
+
+        realm = *execution_context->realm;
+        auto intrinsics = realm->create<Bindings::Intrinsics>(*realm);
+        auto host_defined = make<Bindings::HostDefined>(intrinsics);
+        realm->set_host_defined(move(host_defined));
+    }
+    return *realm;
+}
+
 CSS::CSSStyleSheet* parse_css_stylesheet(CSS::Parser::ParsingParams const& context, StringView css, Optional<URL::URL> location)
 {
     if (css.is_empty()) {
diff --git a/Libraries/LibWeb/CSS/Parser/Parser.cpp b/Libraries/LibWeb/CSS/Parser/Parser.cpp
index f01ec0aa5ff..a5a6d121204 100644
--- a/Libraries/LibWeb/CSS/Parser/Parser.cpp
+++ b/Libraries/LibWeb/CSS/Parser/Parser.cpp
@@ -13,6 +13,7 @@
  */
 
 #include <AK/Debug.h>
+#include <LibURL/Parser.h>
 #include <LibWeb/CSS/CSSStyleDeclaration.h>
 #include <LibWeb/CSS/CSSStyleProperties.h>
 #include <LibWeb/CSS/CSSStyleSheet.h>
@@ -1814,6 +1815,8 @@ bool Parser::is_parsing_svg_presentation_attribute() const
 // FIXME: URLs shouldn't be completed during parsing, but when used.
 Optional<URL::URL> Parser::complete_url(StringView relative_url) const
 {
+    if (!m_url.is_valid())
+        return URL::Parser::basic_parse(relative_url);
     return m_url.complete_url(relative_url);
 }
 
diff --git a/Libraries/LibWeb/CSS/Parser/Parser.h b/Libraries/LibWeb/CSS/Parser/Parser.h
index 468f2ca0ad8..208a01da7f3 100644
--- a/Libraries/LibWeb/CSS/Parser/Parser.h
+++ b/Libraries/LibWeb/CSS/Parser/Parser.h
@@ -521,5 +521,6 @@ CSS::CSSRule* parse_css_rule(CSS::Parser::ParsingParams const&, StringView);
 RefPtr<CSS::MediaQuery> parse_media_query(CSS::Parser::ParsingParams const&, StringView);
 Vector<NonnullRefPtr<CSS::MediaQuery>> parse_media_query_list(CSS::Parser::ParsingParams const&, StringView);
 RefPtr<CSS::Supports> parse_css_supports(CSS::Parser::ParsingParams const&, StringView);
+GC::Ref<JS::Realm> internal_css_realm();
 
 }
diff --git a/Libraries/LibWeb/CSS/StyleComputer.cpp b/Libraries/LibWeb/CSS/StyleComputer.cpp
index 41f5d189615..c8a8426f975 100644
--- a/Libraries/LibWeb/CSS/StyleComputer.cpp
+++ b/Libraries/LibWeb/CSS/StyleComputer.cpp
@@ -309,42 +309,42 @@ struct StyleComputer::MatchingFontCandidate {
     }
 };
 
-static CSSStyleSheet& default_stylesheet(DOM::Document const& document)
+static CSSStyleSheet& default_stylesheet()
 {
     static GC::Root<CSSStyleSheet> sheet;
     if (!sheet.cell()) {
         extern String default_stylesheet_source;
-        sheet = GC::make_root(parse_css_stylesheet(CSS::Parser::ParsingParams(document), default_stylesheet_source));
+        sheet = GC::make_root(parse_css_stylesheet(CSS::Parser::ParsingParams(internal_css_realm()), default_stylesheet_source));
     }
     return *sheet;
 }
 
-static CSSStyleSheet& quirks_mode_stylesheet(DOM::Document const& document)
+static CSSStyleSheet& quirks_mode_stylesheet()
 {
     static GC::Root<CSSStyleSheet> sheet;
     if (!sheet.cell()) {
         extern String quirks_mode_stylesheet_source;
-        sheet = GC::make_root(parse_css_stylesheet(CSS::Parser::ParsingParams(document), quirks_mode_stylesheet_source));
+        sheet = GC::make_root(parse_css_stylesheet(CSS::Parser::ParsingParams(internal_css_realm()), quirks_mode_stylesheet_source));
     }
     return *sheet;
 }
 
-static CSSStyleSheet& mathml_stylesheet(DOM::Document const& document)
+static CSSStyleSheet& mathml_stylesheet()
 {
     static GC::Root<CSSStyleSheet> sheet;
     if (!sheet.cell()) {
         extern String mathml_stylesheet_source;
-        sheet = GC::make_root(parse_css_stylesheet(CSS::Parser::ParsingParams(document), mathml_stylesheet_source));
+        sheet = GC::make_root(parse_css_stylesheet(CSS::Parser::ParsingParams(internal_css_realm()), mathml_stylesheet_source));
     }
     return *sheet;
 }
 
-static CSSStyleSheet& svg_stylesheet(DOM::Document const& document)
+static CSSStyleSheet& svg_stylesheet()
 {
     static GC::Root<CSSStyleSheet> sheet;
     if (!sheet.cell()) {
         extern String svg_stylesheet_source;
-        sheet = GC::make_root(parse_css_stylesheet(CSS::Parser::ParsingParams(document), svg_stylesheet_source));
+        sheet = GC::make_root(parse_css_stylesheet(CSS::Parser::ParsingParams(internal_css_realm()), svg_stylesheet_source));
     }
     return *sheet;
 }
@@ -371,11 +371,11 @@ template<typename Callback>
 void StyleComputer::for_each_stylesheet(CascadeOrigin cascade_origin, Callback callback) const
 {
     if (cascade_origin == CascadeOrigin::UserAgent) {
-        callback(default_stylesheet(document()), {});
+        callback(default_stylesheet(), {});
         if (document().in_quirks_mode())
-            callback(quirks_mode_stylesheet(document()), {});
-        callback(mathml_stylesheet(document()), {});
-        callback(svg_stylesheet(document()), {});
+            callback(quirks_mode_stylesheet(), {});
+        callback(mathml_stylesheet(), {});
+        callback(svg_stylesheet(), {});
     }
     if (cascade_origin == CascadeOrigin::User) {
         if (m_user_style_sheet)