This was was cloned from bug 16351 as part of operation convergence.
Originally filed: 2012-03-13 23:33:00 +0000

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 #0   contributor@whatwg.org                          2012-03-13 23:33:00 +0000 
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Specification: http://www.whatwg.org/specs/web-apps/current-work/multipage/elements.html
Multipage: http://www.whatwg.org/C#the-id-attribute
Complete: http://www.whatwg.org/c#the-id-attribute

Comment:
For ease of interoperability with CSS, suggest adding an authoring
recommendation that id attribute values consist of the characters A-Z, a-z,
0-9, _, - and not begin with a dash.

Posted from: 99.113.33.155
User agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.2) Gecko/20100101 Firefox/10.0.2 Iceweasel/10.0.2
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 #1   Simon Pieters                                   2012-03-14 07:49:58 +0000 
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Same with classes, I presume? What about data-* attribute names?

Note that the rule is:

"In CSS, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [a-zA-Z0-9] and ISO 10646 characters U+00A0 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit, two hyphens, or a hyphen followed by a digit."
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