EDIT: indeed, it was introduced in 2023: https://blog.arxiv.org/2023/12/21/accessibility-update-arxiv...
Why "experimental" HTML?
Did you know that 90% of submissions to arXiv are in TeX format, mostly LaTeX? That poses a unique accessibility challenge: to accurately convert from TeX—a very extensible language used in myriad unique ways by authors—to HTML, a language that is much more accessible to screen readers and text-to-speech software, screen magnifiers, and mobile devices. In addition to the technical challenges, the conversion must be both rapid and automated in order to maintain arXiv’s core service of free and fast dissemination.
> 1. TeX has many advantages that make it ideal as a format for the archives: It is plain text, it is compact, it is freely available for all platforms, it produces extremely high-quality output, and it retains contextual information.
> 2. It is thus more likely to be a good source from which to generate newer formats, e.g., HTML, MathML, various ePub formats, etc. [...]
Not that I disagree with the effort and it surely is a unique challenge to, at scale, convert the Turing complete macro language TeX to something other than PDF. And, at the same time, the task would be monumentally more difficult if only the generated PDFs were available. So both are right at the same time.