Hmmm... fragmentation. I was looking at that in Chrome a while back and support was so-so. So for the heck of it, I hit the css3test.com site with the latest versions of Firefox, Edge, Chrome and Safari. Firefox and Safari managed a 67% rating while Edge and Chrome managed 63%. (finer grained results were Firefox passing 2233 tests, Safari passing 2207 tests, Chrome passing 2112 tests and Edge passing 2110 tests out of 3435 on 819 features.
As for the ePub3 specification, it would appear that many of the features in the latest
CSS snapshot or even the
Fragmentation module from 2108 are not fully supported. Though I do admit to a certain level of amusement when reading "CSS Fragmentation Module Level 3—Breaking the Web, one fragment at a time"