When you try to alter the style of a calendar, in order to be able to read the text more clearly, if making the style of the date readable turns the title into garbage (because there's no semantic information, only they happen to share the same visual class), then someone is going to have a hard time using your website because of your decisions not taking into account their needs.

(This is a slightly tweaked real life example; it was adjusting the style for printing so that the calendar was accessible offline.)