[callahad]

s/subset of/overlapping set with/

AMPHTML whitelists a handful of HTML tags, but adds a bunch of its own custom elements, like <amp-instagram>, <amp-img>, and <amp-ad> on top. It's neither a subset nor a superset of HTML in the common understanding of those terms.

[cramforce]

Custom elements are a thing in HTML. Using them doesn't make it a superset. http://w3c.github.io/webcomponents/spec/custom/