INVALID_CSS_DECLARATION doesn't seem to ever get thrown.

[flukeout]

We've got the following in our strings.js file...

"INVALID_CSS_DECLARATION": "<p><em data-highlight='[[cssDeclaration.start]],[[cssDeclaration.end]]'>This</em> CSS declaration never closes.</p>",

I'm guessing it's an unclosed "block" of rules that is missing a }?

Neither HTMLParser.js not CSSParser.js ever throws it. Do you know what it's about / for @Pomax?

[flukeout]

Alternatively, was is it meant to catch this error? (Missing ; at end of rule)

Nothing is currently catching that ^

[Pomax]

if it's never used, let's rip it out.