I could be convinced to require a space between "10%" and "foo.jpg" as well. The reason why I want to make the space mandatory between "100x100" and "foo.jpg" is because I consider digits and letters to be roughly in the same category. A file name is unlikely to start with a "%", but it may well start with a "100". But more importantly once we add units it definitely needs to be mandatory - otherwise it is unclear whether "100x100pxfoo.jpg" should be "foo.jpg" at size 100x100 or "pxfoo.jpg". A space would resolve this ambiguity. Well, mostly. It could still be "px foo.jpg". :)