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In the current version of the HTML living standard, there's a <div class=impl> wrapper around just the headers of 6 sections: 2.4.1 Common parser idioms 4.8.14.1 Authoring 4.10.22.1 Introduction 7.1.6.1 Event handlers 10.4.2.1 Authors 13.1 Writing XHTML documents So when the reader chooses the alternate stylesheet "Highlight implementation-only prose", there's a highlight on just the header-line of those 6 sections. It's not clear that this has any significance, so I'm guessing it's a mistake.
Most are intentional (they're because the subsequent subsections are all marked as well, so when you remove the class=impl sections, you'd be left with a section containing just a single subsection, which looks weird). These markings are mainly used for developers.whatwg.org, I'd forgotten we even exposed the style sheet. I fixed the ones that were no longer right. Thanks.
Checked in as WHATWG revision r7779. Check-in comment: tweak some class=impl markings http://html5.org/tools/web-apps-tracker?from=7778&to=7779