The thing is, it's not just the HTML standard. It's also all the standards it references. And all the standards they reference, and all the standards those standards reference, ad infinitum.
For example, HTML 5 references SVG 2 which references CSS 2 which references Unicode and XML 11. Or, to go the same route, HTML 5 references SVG 2 which references CSS 2 which references CC.1:2004-10 (Profile version 4.2.0.0) Image technology colour management which references (normative) ISO/IEC 646:1991, Information technology — ISO 7-bit coded character set for information interchange, IEC 61966-2-1 (1999-10), Multimedia systems and equipment — Colour measurement and management — Part 2-1: Colour management — Default RGB colour space — sRGB and TIFF 6.0 Specification, Adobe Systems Incorporated among other things.
Yes, some of those overlap (as many standards will reference many the same standards), but the number of those standards is definitely non-trivial. Some of them you can probably pull in as system libraries or external libraries. The question is, how many?
Edit: and some of them are definitely not relevant to the web, but how would you know until you read through the spec that references it, and through the referenced spec to find and understand the relevant bits?