LibWeb: Don't construct stylesheet when modifying CSSStyleSheet rules

Previously, `CSSStyleSheet.replace()` and `CSSStyleSheet.replaceSync()`
parsed the given CSS text into a temporary stylesheet object, from
which a list of rules was extracted. Doing this had the unintended
side-effect that a fetch request would be started if the given CSS text
referenced any external resources. This fetch request would cause a
crash, since the temporary stylesheet object didn't set the constructed
flag, or constructor document. We now parse the given CSS text as a
list of rules without constructing a temporary stylesheet.

Libraries/LibWeb/CSS/CSSStyleSheet.cpp

@@ -1,7 +1,7 @@
 /*
  * Copyright (c) 2019-2022, Andreas Kling <andreas@ladybird.org>
  * Copyright (c) 2022-2024, Sam Atkins <sam@ladybird.org>
- * Copyright (c) 2024, Tim Ledbetter <timledbetter@gmail.com>
+ * Copyright (c) 2024-2025, Tim Ledbetter <tim.ledbetter@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -207,8 +207,7 @@ GC::Ref<WebIDL::Promise> CSSStyleSheet::replace(String text)
         HTML::TemporaryExecutionContext execution_context { realm, HTML::TemporaryExecutionContext::CallbacksEnabled::Yes };
 
         // 1. Let rules be the result of running parse a stylesheet’s contents from text.
-        auto parsed_stylesheet = parse_css_stylesheet(make_parsing_params(), text);
-        auto& rules = parsed_stylesheet->rules();
+        auto rules = CSS::Parser::Parser::create(make_parsing_params(), text).parse_as_stylesheet_contents();
 
         // 2. If rules contains one or more @import rules, remove those rules from rules.
         GC::RootVector<GC::Ref<CSSRule>> rules_without_import(realm.heap());
@@ -240,8 +239,7 @@ WebIDL::ExceptionOr<void> CSSStyleSheet::replace_sync(StringView text)
         return WebIDL::NotAllowedError::create(realm(), "Can't call replaceSync() on non-modifiable stylesheets"_string);
 
     // 2. Let rules be the result of running parse a stylesheet’s contents from text.
-    auto parsed_stylesheet = parse_css_stylesheet(make_parsing_params(), text);
-    auto& rules = parsed_stylesheet->rules();
+    auto rules = CSS::Parser::Parser::create(make_parsing_params(), text).parse_as_stylesheet_contents();
 
     // 3. If rules contains one or more @import rules, remove those rules from rules.
     GC::RootVector<GC::Ref<CSSRule>> rules_without_import(realm().heap());
@@ -250,7 +248,7 @@ WebIDL::ExceptionOr<void> CSSStyleSheet::replace_sync(StringView text)
             rules_without_import.append(rule);
     }
 
-    // 4.Set sheet’s CSS rules to rules.
+    // 4. Set sheet’s CSS rules to rules.
     m_rules->set_rules({}, rules_without_import);
 
     return {};

Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -118,6 +118,22 @@ Vector<Rule> Parser::parse_a_stylesheets_contents(TokenStream<T>& input)
     return consume_a_stylesheets_contents(input);
 }
 
+GC::RootVector<GC::Ref<CSSRule>> Parser::parse_as_stylesheet_contents()
+{
+    auto raw_rules = parse_a_stylesheets_contents(m_token_stream);
+    GC::RootVector<GC::Ref<CSSRule>> rules(realm().heap());
+    for (auto const& raw_rule : raw_rules) {
+        auto rule = convert_to_rule(raw_rule, Nested::No);
+        if (!rule) {
+            log_parse_error();
+            continue;
+        }
+        rules.append(*rule);
+    }
+
+    return rules;
+}
+
 // https://drafts.csswg.org/css-syntax/#parse-a-css-stylesheet
 GC::Ref<CSS::CSSStyleSheet> Parser::parse_as_css_stylesheet(Optional<::URL::URL> location, Vector<NonnullRefPtr<MediaQuery>> media_query_list)
 {

Libraries/LibWeb/CSS/Parser/Parser.h

@@ -100,6 +100,7 @@ public:
     Vector<Descriptor> parse_as_list_of_descriptors(AtRuleID);
     CSSRule* parse_as_css_rule();
     Optional<StyleProperty> parse_as_supports_condition();
+    GC::RootVector<GC::Ref<CSSRule>> parse_as_stylesheet_contents();
 
     enum class SelectorParsingMode {
         Standard,