For anyone coming along and looking for an applied example, I’ve added an accessibility-friendly scene break example to the sample project in the thread about adding alt text to images.
I would also add that in my testing, the “hr” method may not be as well supported as this web page indicates. Many screen readers will not announce it verbally and just skip right over it, making it as bad as the empty line approach. Plus, the methods described on this page for styling HRs to present themselves as something other than an actual screen-width horizontal rule, do not actually work in a number of ebook readers. You will just get the horizontal rule—which while perhaps not the worst way to cut scenes, certainly isn’t conventional.
I therefore think using an <img ... alt="Scene break" /> type construct is probably the best all around. For those that would prefer a text-based separator instead, such as * * *, or even a custom font symbol, using a 1x1 pixel transparent PNG for the separator, so you get the alt attribute, and the CSS to insert the text itself, might be all around the best option. Using images themselves do have the drawback of easily breaking in different ebook reader themes (black symbols on very dark grey backgrounds in dark mode, for example).