The CSS is generated by a library and not directly under calibre control. In theory the library could be patched, but that's rather more effort than it is worth. Why does the lack of a semi-colon cause problems when using a regex? You should use multi-line mode, for example,
(?m)some-expression.*;?$
will match a css declarationregrardless of whether it ends with a semi-colon or not.
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