In its specifications and process, W3C seeks a balance between
stability (for interoperability) and flexibility (to accommodate
evolution).
HTML 5 has a number of extensibility mechanisms, but none yet that
satisfies the requirement XML namespaces was designed to address of
decentralized extensibility - allowing parties to include their own
elements or attributes in content without risk of name collisions
(whether those names are the result of a consensus process or not).
An HTML Working Group issue on decentralized extensibility is
still open and unresolved. The text/html serialization of HTML5
contains extensibility points but does not provide a decentralized
mechanism.
One of the open questions is what extensibility mechanism can be used
such that, when it is parsed from either the HTML or XML
serialization, the DOM that results is the same (see HTML Design
Principles section 3.5).
