Forum Moderators: not2easy
The good news is yes, it is fully accessible, because it merely runs a replace on already existing tags within your document (e.g. div#content h4) with full specificity abilities.
The W3C have recommended it as a viable solution to wanting specific, anti-aliased text. While seemingly unaffected by browser text size, it actually is - however, controlled by javascript, a refresh is required. Which is fine for actual accessibility, seeing as such people would by default have their text size at a larger setting (thus, the h4 would be at a larger size -before- being replaced, and would replace -at that- larger size.)
It also degrades beautifully - without javascript enabled, or flash enabled, the normal h4s will display with your existing CSS.