Allow CSSImportRule.styleSheet to be null

[amoseui]

WebKittens

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Title of the spec

CSS Object Model (CSSOM)

URL to the spec

https://drafts.csswg.org/cssom/#the-cssimportrule-interface

URL to the spec's repository

https://github.com/w3c/csswg-drafts

Issue Tracker URL

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Explainer URL

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TAG Design Review URL

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Mozilla standards-positions issue URL

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WebKit Bugzilla URL

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Radar URL

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Description

CSSImportRule.styleSheet became nullable in https://github.com/w3c/csswg-drafts/pull/8773 from the issue https://github.com/w3c/csswg-drafts/issues/8608. The behavior of @import had not been matched with CSSOM CSSImportRule.

https://drafts.csswg.org/css-cascade-5/#at-import:

Additionally, if a blocks the application of the imported style sheet, the UA must not fetch the style sheet (unless it is loaded through some other link) and must return null for the import rule’s CSSImportRule.styleSheet value (even if it is loaded through some other link).

It was fixed as following.

https://drafts.csswg.org/cssom/#the-cssimportrule-interface

The styleSheet attribute must return the associated CSS style sheet, if any, or null otherwise.

NOTE: An @import at-rule might not have an associated CSS style sheet (e.g., if it has a non-matching supports() condition).

[annevk]

My main worry here would be that the lifetime of these objects is not carefully defined and as such when they return null and non-null can end up varying in edge cases.