[Quoth]

The ePub2 uses a subset of the HTML a browser can use. Lots of HTML that works in a browser will not work in an eink Kindle (even PW3) or eink ePub based ereader.

I blame the Western Roman/Latin centric nature of the design of the original mobi and epub.

Some ereaders still in use can't do Hebrew, Arabic, Korean, Cyrillic, Horizontal Japanese, Horizontal Chinese and most other writing systems, basically only Roman-Latin and some Greek (really for maths & science, not Greek language). Linux supported all these before the first Amazon Kindle came out, which used Linux.

Hardly anything other than a few apps support epub3. Also some apps only partially support it.

So for non-mainstream western languages, it's only newer ereaders. For vertical text, an app or browser on an Android or Apple Tablet, not an eink ereader as it might not work.