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The rule a.foo:before:hover { color: red } is invalid, while the validator marks it as beeing valid and incorrectly pretty prints it as a.foo:hover:before { color : red; }.
Another test case is P:first-letter P.c { color: red; } which is also not allowed.
See on http://qa-dev.w3.org:8001/
The rule a.foo:before:hover { color: red } passes validation, both using CSS 2.1 profile and using CSS 3 profile. So if the bug was fixed, it has now reappeared. I noticed this when checking why p:first-line:before passes, despite being invalid. It does not pass if the two-colon syntax for pseudo-elements is used, i.e. p::first-line::before.