[css-values-4] <position> is a lie

[nox]

The specification for <position> states that the 3-values syntax from background-position is generically forbidden from it.

This contradicts user agents in the wild, which all accept this supposedly-forbidden syntax everywhere <position> is used, such as in:

• mask-position;
• basic shapes' positions;
• -radial-gradient() position.

[tabatkins]

Yes, it's a recent decision (the April f2f meeting) to try and limit the usage of 3-value position as much as possible. We'll try and remove support for it from places where we can, and amend the definitions to explicitly use the 3-value-included production when proven necessary.

[nox]

@tabatkins Can I assume that the use sites I mentioned will be moved to <bg-position>?

[tabatkins]

As I said, if it proves necessary (that is, if compat prevents us from restricting them to the new <position>).

[nox]

Mmh, interesting, the 3-values syntax cannot be ambiguous in those sites, so why try to restrict them?

[tabatkins]

Because it's a learning hazard. The 3-value syntax is basically a legacy mistake; there's no good reason to have it at all, and if we could remove it from every usage site, we would.

[nox]

Is there any intent from UAs to unship that syntax from the usage sites you have in mind?

[tabatkins]

We're going to attempt that, yes.

[ewilligers]

#2140 mentions progress in unshipping.