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LibWeb: Implement time-traveling inheritance for CSS font-size
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When setting `font-family: monospace;` in CSS, we have to interpret
the keyword font sizes (small, medium, large, etc) as slightly smaller
for historical reasons. Normally the medium font size is 16px, but
for monospace it's 13px.

The way this needs to behave is extremely strange:
When encountering `font-family: monospace`, we have to go back and
replay the CSS cascade as if the medium font size had been 13px all
along. Otherwise relative values like 2em/200%/etc could have gotten
lost in the inheritance chain.

We implement this in a fairly naive way by explicitly checking for
`font-family: monospace` (note: it has to be *exactly* like that,
it can't be `font-family: monospace, Courier` or similar.)
When encountered, we simply walk the element ancestors and re-run the
cascade for the font-size property. This is clumsy and inefficient,
but it does work for the common cases.

Other browsers do more elaborate things that we should eventually care
about as well, such as user-configurable font settings, per-language
behavior, etc. For now, this is just something that allows us to handle
more WPT tests where things fall apart due to unexpected font sizes.

To learn more about the wonders of font-size, see this blog post:
https://manishearth.github.io/blog/2017/08/10/font-size-an-unexpectedly-complex-css-property/
This commit is contained in:
Andreas Kling 2025-02-25 11:47:03 +01:00 • committed by Andreas Kling
commit b4e47f198a
Notes: github-actions[bot] 2025-02-25 22:56:32 +00:00
14 changed files with 241 additions and 106 deletions
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2
Libraries/LibWeb/CSS/StyleComputer.h
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@ -183,6 +183,8 @@ public:
static CSSPixelFraction absolute_size_mapping(Keyword);
RefPtr<Gfx::FontCascadeList const> compute_font_for_style_values(DOM::Element const* element, Optional<CSS::Selector::PseudoElement::Type> pseudo_element, CSSStyleValue const& font_family, CSSStyleValue const& font_size, CSSStyleValue const& font_style, CSSStyleValue const& font_weight, CSSStyleValue const& font_stretch, int math_depth = 0) const;
[[nodiscard]] RefPtr<CSSStyleValue> recascade_font_size_if_needed(DOM::Element&, Optional<CSS::Selector::PseudoElement::Type> pseudo_element, CascadedProperties&) const;
void set_viewport_rect(Badge<DOM::Document>, CSSPixelRect const& viewport_rect) { m_viewport_rect = viewport_rect; }
enum class AnimationRefresh {