[cssom] Serialization of NULL in string

[upsuper]

The algorithm to serialize a string has a separate clause for NULL character which says:

If the character is NULL (U+0000), then the REPLACEMENT CHARACTER (U+FFFD) escaped as code point.

However, this doesn't match behavior of any browser.

Firefox and Chrome serialize NULL character to U+FFFD without escaping it as code point. Edge strips anything after NULL, and Safari serializes it as the character escaped as code point (so \0).

I guess the spec means to do what Firefox and Chrome are currently doing, so probably we should fix the spec text?

[zcorpan]

Yes, it should be a literal U+FFFD.

061ede6 intended to fix this but only did so for identifiers, not strings.