[whatwg] [WA1] lang and xml:lang

Hi,
Web apps currently states [1]:
# Authors should not use the lang attribute in XML documents. Authors
# should instead use the xml:lang attribute.

Is there any reason for not making that "must not"?  The only reason 
someone would ever have for using lang instead of xml:lang in XHTML is 
when serving it as text/html, which is strictly forbidden in this 
version.  It should be stated that lang is for HTML only and xml:lang is 
for X(HT)ML only.

I think the heading for the attribute defintion should be updated to 
include xml:lang as well.

[1] http://www.whatwg.org/specs/web-apps/current-work/#lang

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Lachlan Hunt
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