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CSS Working Group Editor Drafts
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[cssom-1] Serialization for declaration block with variable reference in shorthand may not be useful #2515

Open upsuper opened 6 years ago

upsuper commented 6 years ago

The serialization for declaration block with variable reference in shorthand while some longhand is set differently can lead to undesired result, i.e. it would produce non-idempotent result.

For example, for a declaration block like

margin: var(--foo);
margin-top: 10px;

in the current algorithm would be serialized to something like

margin-right: ;
margin-bottom: ;
margin-left: ;
margin-top: 10px;

which apparently doesn't have the same meaning as before. This is unfortunate.

We should probably change the serialization algorithm to that, when we find a longhand which is expanded from some shorthand with variable reference, the shorthand reference is always serialized. If there are longhands that have been changed from the expansion of shorthand (like margin-top in the case above), the longhands are not added to already serialized, so that their value is still preserved and will be serialized later.

The above algorithm should works fine with declaration block from parsing, since the parsing result uses specified order, and thus any update to longhand which was expanded from a shorthand would always come after the shorthand. However, there is currently some problem with CSSOM setters, because in the spec they still set in place rather than append.

For that, we've resolved in #1898 (comment) that setters should append rather than set in place, so this is probably not a problem anyway. We may still want that change to happen first, though.

cc @csnardi

upsuper commented 5 years ago

It becomes complicated again that #2924 makes it not append, but just has some constraints...

tabatkins commented 3 years ago

Commenting to not that legacy shorthands (which serialize to the non-legacy shorthands that replaced them) are a subset of this issue, but are handled slightly differently in CSSOM, so we should make sure they're solved properly at the same time as this.

css-meeting-bot commented 3 years ago

The CSS Working Group just discussed [cssom-1] Serialization for declaration block with variable reference in shorthand may not be useful.

The full IRC log of that discussion <dael> Topic: [cssom-1] Serialization for declaration block with variable reference in shorthand may not be useful
<dael> github: https://github.com/w3c/csswg-drafts/issues/2515
<dael> Rossen_: Do we have the right people?
<dael> Rossen_: Doubt we have xidorn. Can anyone represent this?
<dael> TabAtkins: This was put on by fantasai, presumably b/c needs to be addressed. not certain we need to address on a call. It's for me and emilio to work on
<dael> Rossen_: I'm fine moving forward if we don't need group consensus.
<dael> TabAtkins: Not yet. May come back later.