HTML5.1 currently has a definition  of the main content that conflicts with the main extension spec 
request that the definition in HTML5.1 spec be removed and someone can then step forward to champion the definition currently residing in HTMl5.1 as an extension spec.
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satisfied with this response, please change the state of this bug to CLOSED. If
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Change Description: The main extension spec has been folded in.
Rationale: It's a good feature to support.