Arguably, would the web exist today as it is if not for robustness principle that also leads to "almost impossible for anyone other than one of the giants to create a working implementation"
Similarly for JS, strict mode did not ruin the internet. Sure, there is a lot of old JS code around that may or may not run in strict mode. That is not because it is any harder to write correct JS in strict mode than it is in "sloppy" mode, but the "robustness principle" has just burdened us with a lot of legacy crap that wouldn't otherwise exist. I would argue that the web would be better if JS had been strict from the start.
One of the possible definitions is a mode of HTML that was forced to be strictly validating by an XML parser. Adoption of this was hurt by the fact that XHTML-delivered-as-XHTML didn't work correctly in the dominant browser of the time. But HTML5 also delivered a fully-specified parser that explained how to handle tag soup correctly, which meant the problem trying to be solved here (having to parse HTML files into a DOM) was solved in a different way.
Another possible definition is the XHTML 2.0 definition, which tried to rip out a lot of HTML features and replace them with more semantic definitions and was largely an effort that was entirely divorced from reality (to the point of driving out the web browser implementors entirely from the working group).
You don't need closing tags for a specification to be strict. As long as you define the parsing rules rigorously as HTML5 does, there is little benefit in XML compliance and there is in a well defined non-XML standard as it is mostly backwards compatible.
- strict mode is a much more local opt-in, it’s difficult to opt others into strict mode, and thus the likelihood of respecting strict mode is high
- js is much less subject to embeddings, with xhtml even back then there was a long tail of CMS type content which would suddenly break the page hard and with no actionable recourse for the viewer
- finally strict mode came with a bunch of benefits e.g. new syntax / constructs, etc, for the person opting into strict mode, but unless you were an XML-head with an entire XML-based processing pipeline, what did XHTML give you? Additional pain when authoring content is about all.
The "impossibility" is not the quirks and ends of HTML - is adding all the 100s of frameworks on top, like 5 accelerated graphic options (Canvas, WebGL, SVG, CSS Animations, WebGPU), all kinds of type-setting-foo including math rendering, real-time communications, 2 optimized-to-death-by-experts languages (JS, WASM), storage options, sandboxes, audio, video, MIDI, and synthesizer capabilities, and so on...