Intent to Ship: CSSPageRule to inherit from CSSGroupingRule
Morten Stenshorne
Contact emails
mste...@chromium.orgExplainer
NoneSpecification
https://drafts.csswg.org/cssom/#the-csspagerule-interfaceSummary
Let CSSPageRule inherit from CSSGroupingRule instead of CSSRule. The spec [1] says that the CSSPageRule interface should inherit from CSSGroupingRule, but Blink inherits from CSSRule instead. In order to support @page margins (CSSMarginRule), CSSPageRule needs to support child rules. [1] https://drafts.csswg.org/cssom/#the-csspagerule-interface [2] https://drafts.csswg.org/css-page-3/#syntax-page-selector
Blink component
Blink>CSSTAG review
NoneTAG review status
Not applicableRisks
Interoperability and Compatibility
Low risk. The one possible issue is if an author uses "instanceof CSSGroupingRule" or "instanceof CSSRule" with a page rule object and makes incorrect assumptions based on that. However, given that this is already shipping in Firefox (and also that in Firefox, even CSSStyleRule inherits from CSSGroupingRule, as the spec says - whereas Blink still doesn't), the risk should be very low.
Gecko: Shipped/Shipping (https://bugzilla.mozilla.org/show_bug.cgi?id=1868215)
WebKit: No signal
Web developers: No signals
Other signals:
WebView application risks
Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?
None
Debuggability
None
Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, ChromeOS, Android, and Android WebView)?
YesIs this feature fully tested by web-platform-tests?
YesFlag name on chrome://flags
NoneFinch feature name
NoneNon-finch justification
There's no way to make such a change behind a runtime feature flag, so this will just have to ship when the code change lands. In the worst case, it will have to be reverted.
Requires code in //chrome?
FalseTracking bug
https://issues.chromium.org/issues/324827379Estimated milestones
| Shipping on desktop | 126 |
| Shipping on Android | 126 |
Anticipated spec changes
Open questions about a feature may be a source of future web compat or interop issues. Please list open issues (e.g. links to known github issues in the project for the feature specification) whose resolution may introduce web compat/interop risk (e.g., changing to naming or structure of the API in a non-backward-compatible way).
NoneLink to entry on the Chrome Platform Status
https://chromestatus.com/feature/5218866421825536?gate=6703446991568896
Mike Taylor
On 5/8/24 3:37 AM, Morten Stenshorne wrote:
Interoperability and Compatibility
Low risk. The one possible issue is if an author uses "instanceof CSSGroupingRule" or "instanceof CSSRule" with a page rule object and makes incorrect assumptions based on that. However, given that this is already shipping in Firefox (and also that in Firefox, even CSSStyleRule inherits from CSSGroupingRule, as the spec says - whereas Blink still doesn't), the risk should be very low.
Noting that the patch should ship to Firefox release channel (126) in about a week, per https://whattrainisitnow.com/calendar/.
Can we request a signal? Or do we have any other indications about their intentions here?
WebKit: No signal
Morten Stenshorne
On 5/8/24 3:37 AM, Morten Stenshorne wrote:
I agree the risk is likely very low, or contained to just a few sites/applications - have you done any investigation to try to find any problematic code examples that you mention (via GitHub search or HTTPArchive)?Interoperability and Compatibility
Low risk. The one possible issue is if an author uses "instanceof CSSGroupingRule" or "instanceof CSSRule" with a page rule object and makes incorrect assumptions based on that. However, given that this is already shipping in Firefox (and also that in Firefox, even CSSStyleRule inherits from CSSGroupingRule, as the spec says - whereas Blink still doesn't), the risk should be very low.
@media screen {
div {
color: hotpink;
}
}
@page {
margin: 1in;
@top-center {
content: "Header";
}
}
</style>
<script>
for (const rule of document.styleSheets[0].cssRules) {
if (rule instanceof CSSGroupingRule) {
// Aha! You're a media rule! (d'oh!)
// ...
}
}
</script>
Noting that the patch should ship to Firefox release channel (126) in about a week, per https://whattrainisitnow.com/calendar/.Gecko: Shipped/Shipping (https://bugzilla.mozilla.org/show_bug.cgi?id=1868215)
Can we request a signal? Or do we have any other indications about their intentions here?
WebKit: No signal
Mike Taylor
Hi Morten -
Appreciate the response. I spent some time looking at
https://github.com/search?q=%22instanceof+CSSGroupingRule%22+language%3AJavaScript&type=code&l=JavaScript
as well as "instanceof CSSRule" earlier, but didn't see anything
immediately worrying (granted, it's hard to predict perfectly how
the tooling-related scripts might handle sites in the wild that
might ship the problematic patterns).
LGTM1 to ship.
Chris Harrelson
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TAMURA, Kent
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Software Engineer, Google