Hmm, that’s right. Looks like this is indeed more complicated to get right than I initially thought. 
One reason for my confusion is that validity against a schema is not the same thing as processor compatibility. To validate a 1.0 style you’ll need a 1.0 schema but you can use this style with a processor that implements 1.8. (At least, according to what @cormacrelf writes here.) So, concerning the processor version="1.0" indicates the minimal requirements, but for validation you’ll need an exact match. Right?
This also means that we cannot (or should not) simply update the version attribute in all existing styles to version="1.1".
But: Do you think it will be necessary to validate against 1.0 once 1.1 is out? If yes, what about relying on git checkout tags/v.1.0.1 -b master for these cases.