[XBL] - please remove reference to HTML5

This comment comes from the W3C Staff.

Please remove the references to the HTML5 specification, at least in  
the version published by the W3C. To be clear, this is not a negative  
comment on HTML5 -- the reference is ok, but a W3C specification  
should not rely on normative behaviour described in an unstable  
specification.

For example:

 > Security contexts are (or will be) described in the HTML5  
specification. [HTML5]

Hopefully new work on HTML at the W3C can allow such a reference in  
the near future.

Dean