That it's locked at where it was over twenty years ago (modulo Ian Hickson's additions ie article, header, section, etc) with everything around it (CSS, JS) having been bloated ad absurdum to make up for HTML's lack of evolution. Yes I know we want to call it separation of presentation from ... whatever, but looks more like an organizational stalemate and self-inflicted problem to me. Today, WHATWG HTML the markup language specifies a vocabulary for casual academic publishing and in prose rather than formal fashion; when in reality it's used for messages and discussion threads, ads of all kinds, CVs, press articles, forms workflows, and countless other kinds of text. To capture such uses was the goal of W3C's XML initiative in 1997 for bringing new vocabularies to the web; but they obsessed over "meta" and enterprise techniques and only brought SVG and MathML. Subsetting XML from SGML was an entirely pointless 20+ years detour, and today SGML still remains the only game in town for formally capturing and evolving HTML. Hickson's/WHATWG's HTML spec OTOH is already fossilized and error-prone to update with new elements.