Hehe, purity is one helluva drug!
For some reason, this makes me think of SVG's foreignObject tag that gives a well-defined way to add elements into an SVG document from an arbitrary XML namespace. Display a customer invoice in there, or maybe a Wayland protocol. The sky's the limit!
On the other hand, HTML had so many loose SVG tags scattered around the web that browsers made a special case in the parser to cover them without needing a namespace.
And we all know how that played out.
Posted from an xhtml foreignObject on my SVGphone