Comment by tannhaeuser

Why is this linked now? There's no new release or milestone at this time.

Citing my comment from when this was new about eight months ago:

> Even more unfortunate is that this change has already spilled to derived standards such as EPUB3 which hence makes existing EPUB3 content using compound headings going back to 2011 invalid, and EPUB3 writers lacking a tool for actually verifying what readers can support (epubcheck was blindly updated without consideration for the installed base).

See also the blog [1] about W3C's most recent HTML spec. Lack of HTML backward compat along with gross import of all of CSS without profiles, or paged media requirements and deemphasis of long-standing EPub mechanisms in favor of CSS and JS, and general impression of a low-effort, merely editorial nature really makes Epub's move to W3C questionable but nobody seems to care anyway, sticking with EPub 2 and 3.1 (which is also what Calibre is recommending as target format for conversion).

[1]: https://sgmljs.net/blog/blog2303.html