As the spec points out:
> Note that a pair of keywords can be reordered while a combination of
> keyword and length or percentage cannot. So center left is valid while
> 50% left is not.
This was a bug in our implementation of alternative 2 of css-values-3,
resulting in the following CSS failing to be parsed:
`background-position: center right;`
This commit fixes the issue as part of an update of the parsing to
css-values-4. As far as I can tell, the grammar is equivalent - but
simpler to implement due to the lack of optional values.
The fix for this issue is also as part of alternative 2 parsing in the
new grammar.
Progress towards: #22401
5b9a0e3fe7