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LibWeb: Block rendering until linked stylesheets are loaded

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This commit implements the main "render blocking" behavior for link
elements, drastically reducing the amount of FOUC (flash of unstyled
content) we subject our users to.

The document will now block rendering until linked style sheets
referenced by parser-created link elements have loaded (or failed).

Note that we don't yet extend the blocking period until "critical
subresources" such as imported style sheets have been downloaded
as well.
This commit is contained in:
Andreas Kling 2025-02-27 15:30:26 +01:00 • committed by Andreas Kling
commit 043e96946f
Notes: github-actions[bot] 2025-02-27 20:37:32 +00:00
5 changed files with 52 additions and 8 deletions
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9
Libraries/LibWeb/HTML/Parser/HTMLParser.cpp
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@ -1,5 +1,5 @@
/*
* Copyright (c) 2020-2024, Andreas Kling <andreas@ladybird.org>
* Copyright (c) 2020-2025, Andreas Kling <andreas@ladybird.org>
* Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
* Copyright (c) 2023-2024, Shannon Booth <shannon@serenityos.org>
*
@ -30,6 +30,7 @@
#include <LibWeb/HTML/HTMLFormElement.h>
#include <LibWeb/HTML/HTMLHeadElement.h>
#include <LibWeb/HTML/HTMLHtmlElement.h>
#include <LibWeb/HTML/HTMLLinkElement.h>
#include <LibWeb/HTML/HTMLScriptElement.h>
#include <LibWeb/HTML/HTMLTableElement.h>
#include <LibWeb/HTML/HTMLTemplateElement.h>
@ -781,6 +782,12 @@ GC::Ref<DOM::Element> HTMLParser::create_element_for(HTMLToken const& token, Opt
// 9. Let element be the result of creating an element given document, localName, given namespace, null, is, and willExecuteScript.
auto element = create_element(*document, local_name, namespace_, {}, is_value, will_execute_script).release_value_but_fixme_should_propagate_errors();
// AD-HOC: Let <link> elements know which document they were originally parsed for.
// This is used for the render-blocking logic.
if (local_name == HTML::TagNames::link && namespace_ == Namespace::HTML) {
as<HTMLLinkElement>(*element).set_parser_document({}, document);
}
// 10. Append each attribute in the given token to element.
token.for_each_attribute([&](auto const& attribute) {
DOM::QualifiedName qualified_name { attribute.local_name, attribute.prefix, attribute.namespace_ };