You are of course correct that ePub nowadays doesn’t mandate a given version of CSS (though earlier versions did!), but that doesn’t matter in this case: it’s non-conforming according to even CSS level 1 (1996), per https://www.w3.org/TR/REC-CSS1-961217#forward-compatible-par...

> illegal values, or values with illegal parts, are treated as if the declaration weren't there at all

So a conforming implementation would ignore that max-width property declaration, not raise an error.

And those earlier versions of ePub which defined a required subset of given CSS standards? The forwards-compatible parsing rules were part of their subset.