[gregable]

It is just a subset of HTML (as is amp). Most email providers that accept HTML formats already only accept a subset of HTML. This is just a different, more formally defined, subset.

[nixpulvis]

I mean, HTML is just XML... these subsets are in many ways their own language.

[dingaling]

HTML is an application of SGML / ISO8879. It defines its own domain and that's the scope of it. It's a leaf.

XML is a subset of SGML and can have further subsets and implementing applications I.e. it is a branch. As noted above, XHTML was a leaf on this branch.

So they are related but not equivalent.

[seabrookmx]

It's not, technically. XHTML was a formally defined HTML subset of XML but it died.

[nixpulvis]

Fair enough, my point stands though.