Don't know if I'd characterize the situation like that. HTML has changed very, very little over time: there were the new HTML 5 semantic sectioning elements article, main, header, footer, section, and very few additional elements with ever-changing semantics such as summary/details, dialog and menu/menuitem (oddly, a Firefox contribution that took a long time to find its way into other browsers, unlike Chrome's). Note the so-called "HTML5 outlining algorithm" for inferring sections and interpreting heading levels was only removed last year from the spec. In contrast, many more additional elements came in via the foreign vocabularies that were specified using XML ie SVG and MathML.
What utterly changed was CSS and JS, though; you could say CSS was expanded to this absurd extend because HTML was bound to stay the same during W3C and then WHATWG's organizational lock on HTML.
This is true. It was really the whole browser HTML/CSS/DOM/JavaScript platform which was considered locked and frozen by IE. Flash supported JavaScript but through its own independent engine, which meant it could evolve independently of IE. Something similar was envisioned for SVG, which is why it needed its own networking API independent of what the browser provided.
Of course things turned out differently, and SVG became integrated in the HTML rendering engine and browser environment.
What utterly changed was CSS and JS, though; you could say CSS was expanded to this absurd extend because HTML was bound to stay the same during W3C and then WHATWG's organizational lock on HTML.