IE 7 does not support generated content (:before
and :after
pseudo-elements) at all. So the problem has nothing do with support to the CHECK MARK “✓” character, as you can see by testing with that character in HTML content (written as ✓
if needed) and with generated content using some common character, say content: 'X'
(it won’t work either).
If IE 7 is relevant, consider adding the symbol into actual document content instead of generating it with CSS. You might consider doing this with JavaScript if it needs to be dynamic (in a browser).