[cssom] "Serialize a CSS declaration" does not properly serialize property names with special characters

[luizgfranca]

On https://drafts.csswg.org/cssom/#serialize-a-css-declaration the property name string is appended directly onto the final string, but this behavior would break the redefinition of the same style with e.style = e.style.cssText when there are custom properties with special characters.

This is demonstrated by the WPT test: https://wpt.fyi/results/css/cssom/variable-names.html, from which the current specified behavior seems to disagree.

Should we maybe apply the algorithm from https://www.w3.org/TR/cssom-1/#serialize-an-identifier to the property name?

[cdoublev]

Also see #12258.