Bug 276223 – The `initial-value` descriptor when registering Custom Properties can be optional

• Component: CSS
• Status: NEW

[Bramus]

# Demo

The following page uses a CSS Space Toggle to color the page green or red. It should color the page green when the engine used has `@property` support and red when the engine used does not support `@property`.

URL: https://codepen.io/bramus/pen/bGjQVEm

Code:

```css
@property --registered {
  syntax: '*';
  initial-value: ;
  inherits: false;
}

body {
  --bg-when-supported: var(--registered) green;
  --bg-when-not-supported: var(--registered, red);
  background: var(--bg-when-supported, var(--bg-when-not-supported));
}
```

# Results

Actual Result: The page colors red in Safari
Expected Result: The page colors green in Safari
Other browsers: The page colors green in Chrome 119+ [^1] and Firefox Nightly.

# The problem

WebKit seems to ignore this part of the spec [^2]

> If the value of the syntax descriptor is the universal syntax definition, then the initial-value descriptor is optional. If omitted, the initial value of the property is the guaranteed-invalid value.

[^1]: This commit enabled that: https://chromium-review.googlesource.com/c/chromium/src/+/4890262
[^2]: https://drafts.css-houdini.org/css-properties-values-api/#initial-value-descriptor

[Karl Dubost]

Created attachment 471833 [details]
rendering in safari, firefox, chrome

[Radar WebKit Bug Importer]

<rdar://problem/131288198>

[Karl Dubost]

tested in 
Safari Technology Preview  198          20619.1.20.7
Firefox Nightly            129.0a1       12924.7.4
Google Chrome Canary       128.0.6582.0  6582.0

[Ahmad Saleem]

In Source/WebCore/css/parser/CSSVariableParser.cpp:

From 'CSSVariableParser::parseDeclarationValue' & 'CSSVariableParser::parseInitialValueForUniversalSyntax' functions, removing this bit - fixes the test cases but WPT test - which was added by Blink / Chromium, Safari Technology Preview 221 is already passing, so we are missing coverage here since codepen demo is failing yet WPT is passing:

if (range.atEnd())
        return nullptr;

^ Remove this from both functions and we pass this codepen demo.

[Ahmad Saleem]

Created attachment 475577 [details]
Reference Screenshot (Patched Minibrowser - WebKit ToT (296257@main))

[Ahmad Saleem]

With patch, there is no failure or change in result on synced WPT in WebKit repo for `css-variables` and `css-properties-values-api` directories. So no progression or regression.

[Ahmad Saleem]

Pull request: https://github.com/WebKit/WebKit/pull/46899