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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
parent 5146bbe296
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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22
Libraries/LibWeb/HTML/HTMLLinkElement.cpp
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@ -1,5 +1,5 @@
/*
* Copyright (c) 2018-2023, Andreas Kling <andreas@ladybird.org>
* Copyright (c) 2018-2025, Andreas Kling <andreas@ladybird.org>
* Copyright (c) 2021, the SerenityOS developers.
* Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
* Copyright (c) 2023, Srikavin Ramkumar <me@srikavin.me>
@ -497,6 +497,8 @@ void HTMLLinkElement::process_stylesheet_resource(bool success, Fetch::Infrastru
// FIXME: 2. Decrement el's node document's script-blocking style sheet counter by 1.
// 7. Unblock rendering on el.
unblock_rendering();
m_document_load_event_delayer.clear();
}
@ -526,16 +528,30 @@ bool HTMLLinkElement::stylesheet_linked_resource_fetch_setup_steps(Fetch::Infras
// 3. If el's media attribute's value matches the environment and el is potentially render-blocking, then block rendering on el.
// FIXME: Check media attribute value.
if (is_potentially_render_blocking())
block_rendering();
m_document_load_event_delayer.emplace(document());
// 4. If el is currently render-blocking, then set request's render-blocking to true.
// FIXME: Check if el is currently render-blocking.
request.set_render_blocking(true);
if (document().is_render_blocking_element(*this))
request.set_render_blocking(true);
// 5. Return true.
return true;
}
void HTMLLinkElement::set_parser_document(Badge<HTMLParser>, GC::Ref<DOM::Document> document)
{
m_parser_document = document->make_weak_ptr<DOM::Document>();
}
bool HTMLLinkElement::is_implicitly_potentially_render_blocking() const
{
// A link element of this type is implicitly potentially render-blocking if the element was created by its node document's parser.
return &document() == m_parser_document;
}
void HTMLLinkElement::resource_did_load_favicon()
{
VERIFY(m_relationship & (Relationship::Icon));