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[selectors-4] Legacy : pseudo-element cannot appear before type/subclass selectors #8122

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cdoublev opened this issue Nov 22, 2022 · 3 comments
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[selectors-4] Legacy : pseudo-element cannot appear before type/subclass selectors #8122

cdoublev opened this issue Nov 22, 2022 · 3 comments
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Closed Accepted as Editorial Commenter Satisfied Commenter has indicated satisfaction with the resolution / edits. selectors-4 Current Work

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@cdoublev
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cdoublev commented Nov 22, 2022 •
edited

Chrome/FF seems to parse :before#id or :before.class as invalid (instead of valid and matching nothing) but I do not see on which part of the spec this is based.

Pseudo-elements are featureless, and so can’t be matched by any other selector

This does not make them invalid.

Legacy : pseudo-element syntax is defined in <subclass-selector> as a <pseudo-class-selector>.

<compound-selector> = [ <type-selector>? <subclass-selector>* [ <pseudo-element-selector> <pseudo-class-selector>* ]* ]!

Subclass selectors can appear in any order while still selecting the same set of elements, but this is not true for a pseudo-element specified with the legacy single : syntax.

Therefore I assume that :before must be handled as if it has been specified with ::before and matched <pseudo-element-selector>. If so, it may be helpfull to define it in the spec.

A similar confusion occurred to me when parsing a <color-stop-list>, whose syntax requires two color stops but its grammar accepts an implicit color stop defining two stops with the same color.
@tabatkins
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The intent of the additional rule is to say that you can spell a few pseudo-elements with a single colon:

The four Level 2 pseudo-elements (::before, ::after, ::first-line, and ::first-letter) may, for legacy reasons, be represented using the grammar, with only a single ":" character at their start.

I should probably write that out more clearly. I can just merge that into the grammar itself rather than putting it in prose.

tabatkins added a commit that referenced this issue Nov 22, 2022
@tabatkins
[selectors] Move the legacy pseudo-element syntax into the grammar it…
23e725e 
…self, for clarity. No normative change intended. #8122
@tabatkins
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And done.

@cdoublev
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Thanks, I am satisfied and happy that I do not have to implement a custom validation.

@tabatkins tabatkins added Commenter Satisfied Commenter has indicated satisfaction with the resolution / edits. and removed Commenter Response Pending labels Nov 23, 2022
jakearchibald pushed a commit to jakearchibald/csswg-drafts that referenced this issue Jan 16, 2023
@tabatkins @jakearchibald
[selectors] Move the legacy pseudo-element syntax into the grammar it…
1169996 
…self, for clarity. No normative change intended. w3c#8122
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