fix(css-syntax): fix some minor css syntax bugs

[fiji-flo]

Summary

This includes some minor fixes for CSS syntax :

• link <length [0,∞]> to length
• link ! in [ a | b ]!
• prefer main spec for at sytntax rule descriptors

---

How did you test this change?

See https://static.fiji-flo.de/diff-part-2.html

• -webkit-line-clamp/index.json is an example <integer [1,∞]>
• background-position-x/index.json is an example for the ]! fix
• @font-face/ascent-override/index.json is an example for using css-fonts instead of css-fonts-5

[fiji-flo]

@wbamberg are you fine with these? Especially the last one:

• prefer main spec for at sytntax rule descriptors

[dipikabh]

I took a quick look, @fiji-flo. The results in the right column on https://static.fiji-flo.de/diff-part-2.html look good.

To confirm, for "@page/index.json", will the part "Where the includes:" be included in the Formal syntax?

[fiji-flo]

No that's always the whole section of https://developer.mozilla.org/en-US/docs/Web/CSS/@page#formal_syntax.

[dipikabh]

We should fix this in content (I'll make a note) to remove the hand-curated part following https://developer.mozilla.org/en-US/docs/Web/CSS/@page#formal_syntax. <page-body> is no longer relevant, and <pseudo-page> is getting covered in the syntax.