Why a subset of HTML you ask? Well, mostly because web developers suck at their jobs and have loaded the web with a ton of JavaScript no one wants. Can't fault Google for wanting to change that. That part I can support. The less JavaScript the better.

One of the principles of the web, neutral or not neutral is the following one: The less Javascript, libraries included, except for multimedia purposes, the better.

If is not met this requirement, there is the need of a subset. We know that from the ancient DHTML days, remember? Try to understand the Document Object Model, first.

Folks at Google work on Accelerated Mobile Pages (AMP) , a subtle recover of that old AS1 magic we knew in the past when there was the need to have a choice in between WAP and Flash Lite technologies.

Google works on AMP  understanding the media, that is, mobile. The common user of th web, does not care about tech. And developers work on behalf the ordinary and common people, like you and this yours truly.  That was why web standards were made. It's the audience, stupid.

More on the matter, flavoured with pros and cons : The two faces of AMP, JS on AMP Google AMP is Not a Good Thing, et cetera

Today’s average web developer doesn’t have the faintest clue what he’s doing and can’t do it anyway without sixteen libraries and frameworks. Solve that problem and AMP goes away.

via Quirks