Call for review: namespaced element definition

Hey folks,

This pull request adds a new definition of "namespaced element", which lets
us be more explicit about what elements particular rules apply to. This
takes away the ambiguity about whether custom elements are considered HTML
elements (they are).
https://github.com/act-rules/act-rules.github.io/pull/1636/

If you have any comments, please leave a review. This call for review ends
on June 29th.

Kind regards,

-- 
*Wilco Fiers*
Axe-core product owner - Facilitator ACT Task Force - Co-chair ACT-Rules