1972340 - CSS.supports('break-after', 'avoid') returns true despite no support

Status: NEW

(Core :: CSS Parsing and Computation, defect)

Description

[Claas Augner [:claas]]

Background: Firefox doesn't support [page-]break-before/after: avoid (bug 775617).

Expected: CSS.supports('break-after', 'avoid') should return false.

Actual: CSS.supports('break-after', 'avoid') returns true.

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The following all return true, although the features aren't implemented :

CSS.supports('break-before', 'avoid');
CSS.supports('break-after', 'avoid');

CSS.supports('page-break-before', 'avoid'); // legacy
CSS.supports('page-break-after', 'avoid'); // legacy

(page-break-inside: avoid and break-inside: avoid are omitted here, as these are implemented, see bug 685012.)

---

To check, you can also run the MDN BCD Collector tests (which use CSS.supports() to determine support):

• break-before
• break-after
• page-break-before
• page-break-after

Comment 1

[Emilio Cobos Álvarez [:emilio]]

We do support them (as in, parse them), though I think indeed there are rather important bugs in our implementation. E.g. we support them in flexbox afaict (bug 1890238), but not in a lot of other places.

Comment 2

[Claas Augner [:claas]]

TIL the spec for CSS.supports() states:

If (…) value successfully parses according to that property’s grammar, return true.

So it seems CSS.supports() is not actually intended for feature detection?

Earlier I came across comment 3 in bug 775618:

We don't actually implement these values, so we shouldn't parse them, either.

So this makes me wonder why we parse avoid?

Note that comment 2 in bug 1890238 states:

In this patch, we don't handle avoid
break values such as break-before:avoid and break-after:avoid since they are
not handled in other frame types yet.

So I don't think avoid is supported in flexbox either.

Comment 3

[Emilio Cobos Álvarez [:emilio]]

It seems we've parsed page-break-after: avoid at least as far back as 2007: https://hg-edge.mozilla.org/mozilla-central/rev/2eaf4545bec3c303a80c3ddcfabd6eb024eace15

Then where we rejiggered this / renamed them to break-after it just got carried forward.