Maybe because I used inline-block? I'll try simply block.
I also tried DIV and SPAN and none worked. On some readers the text is placed before the main text (like it would have been no tags), on others the text was displayed as a different paragraph, above.
I'll try the suggestions.
SideNote.ePub works on my Sony PRS-T3, but no on my Sony Z3 with fbreader. Since FBreader does not yet support tables, I think I have to search for another eReader for Android.