Thanks everybody.
I'll try your suggestions. I already have a saved search for this, but I seldom use it, because when I type "v" in the search box, the first available option is "vertical-align:top", and "vertical-align:super" in the replace box.
Of course, I understood it was not punctuation, as I use very often the "smarten punctuation" command: punctuation conventions are very complicated in French (always non-breakable spaces with "double" punctuation marks (;:?!,«»), and many ebook makers don't comply with them.
I don't agree with arbitrarily setting fonts size either, for the same reasons as you, eschwartz (though I'm rather a "rational pessimist").
Maybe add a "fix CSS for Kobo" option?
BTW, I also noticed in some ebooks strange errors like 1,5em instead of 1.5em. Maybe the result of a localization madness which often happens in free software.
For instance, Gimp can no longer work properly with UFRaw when it is set to French (probably arithmetical dots converted to commas somewhere
Another example of this madness: with the last version of MKVMerge GUI, when I want to set a stream language to French, the dropdown menu will drop up and open on zza (Zaza; Dimil;..., and (as you cannot type the first letters) I must laboriously navigate through hundreds of local languages before I can reach French. In Windows versions, common languages are on top, but I suppose this was considered imperialist...