Reproducible with fairly recent Emacs 24 build:

<begin CSS>
        // This should have a white bottom border
        // But the way we're running the main border on tab-content, not on nav-tab
        // means that this sits under the border line, and so our white border is invisible
        // to fix this, raise the z-index
</end>

Note single quote on line 2 ruins font-locking.

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Le