The properties to be used are:
Code:
.breakBefore {
page-break-before: always;
break-before: column;
-webkit-column-break-before: always;
break-inside: avoid-column;
-webkit-column-break-inside: avoid;
}
You're right, PocketBook (only under epub3 and with the standart engine) and Colibrio don't honor them, but all others ereaders do (Gitden, Reasily, Lithium, OverDrive, Aldiko Next, BlueFire, InfinityReader, BookFusion (all of them for Android), Thorium (both Windows and Linux versions), Koodo Reader (both Windows and Linux versions), ADE (Windows), IceCream (Windows), Foliate (Linux), Calibre Viewer and the Sigil plugins Readium, Bibi and EpubJSReader. (Of course, I only cited apps and programs with support for epub3)
PocketBook employs RMSDK (ADE Legacy) to display epub2 ebooks and Webkit to display epub3. It must be using and ancient version of webkit because otherwise it should honor page-breaks under epub3. Let's hope that a new version of the app updates the engine.