There doesn't seem to be an EPUB example file on the EPUB main page

[ell1e]

I suggest there should be some old public domain book formatted as a modern EPUB2 and as a modern EPUB3 file available here: https://kb.daisy.org/publishing/docs/epub/ Which shows all the standards and requirements applied, including correct semantic use of chapters, how the navigational structure is expected to look like, the access modes and accessibility summary, and so on. Sorry if this is already linked somewhere, but I can't see it. I suggest it is linked on the page I specified, since that is where people may want to see a complete example to download and unzip as a starting point to see how it's meant to look like as a whole.

[ell1e]

I feel like the hr style="width:0; margin: 0.5em 0; would be a great test case in such a centrally available test file, to also be able to see what reader software actually renders it as intended. I think I've seen at least one that has some sort of custom HTML/CSS handling that isn't a full web engine, so then such tweaks might end up having interesting effects.