2008-05-08 15:20:19: Created issue 'Decentralized extensibility' nickname Dave Orchard owned by David Orchard on product , description 'The HTML5 specification does not have a mechanism to allow decentralized parties to create their own languages, typically XML languages, and exchange them in HTML5 text/html serializations. This would allow languages such as SVG, MathML, FBML and a host of others to be included. At one point, an editors version of the HTML5 specification contained a subset and reformulation of SVG and MathML. Tim Berners-Lee described this incorporation of SVG and MathML without namespaces as horrific and the issue raiser completely concurs with the him.
This issue limits the ability of non-HTML5 working groups to define languages as the languages must be "brought into" the HTML5 language. This dramatically increases the scope of HTML5 and decreases the ability to modularize development of orthogonal languages.
In the end, the problem could result in the text/html serialization rules becoming the standard serialization rules for XML languages, replacing XML itself. This could occur if every decentralized language has a choice between the XML serialization, the text/html serialization or both. In many cases, the language may choose the text/html serialization.
This issue was first raised in August 2007 by Sam Ruby in  and also raised at the recent W3C AC meeting by Tim Berners-Lee 
(member-only link until Tim publishes in a public way)
' non-public [David Orchard]