Just to add a bit more info regarding "special spaces"and davidfor's font tester epub ...
I often use Adobe Garamond as a sideloaded font on my Kobos so I had a look inside the Regular ttf file. As far as I can tell this font does not actually contain the glyphs for any of these enSpace (u2002), emSpace (u2003), thinSpace (u2009), hairSpace (u200A) or narrowNBSP (u202F).
Kepub:
Seems to display characters enSpace and emSpace OK, but the other 3 display as a cross-in-a-box. I'm guessing the kepub reading app is finding the first 2 in one of the other (built-in???) fonts and falling back to them. (It's hard to be sure with spaces which have no distinguishing features
) The last 3 presumably don't occur in any of the fonts being used for fallback.
I had success displaying all 6 "special spaces" with fonts like Times New Roman, Charis SIL, Bookerly and Linux Libertine G. All of these contain all 6 glyphs without needing to fallback to some other device font.
Epub:
I only tested standard epub briefly but it did
seem to correctly display all 6 spaces. Even with Adobe Garamond which doesn't actually contain 5 of the glyphs. I have no idea how the Adobe reading app is "finding", or maybe constructing, the missing glyphs.