How so? I've done this often in the past and the ePub always was able to be compiled. I used Sigil. There are also apps that will strip out CSS that is internally resident in the HTML file(s). If a class is not defined, it should default the element to the default value. The real issue, and a royal PITA, is when the publisher uses DIV tags and P tags for chapters headings, sub headings, and pretty much everything else. In that case, it is probably more trouble than it is worth to correct that mess.