[jQuery] How to get font size in pixels?

[Sam Collett]

When I use $("h1").css("font-size") it differs across browsers.
Firefox returns it the way I want (in pixels), but Internet Explorer
it returns it as returned from the stylesheet (so it could be % or
em). How can I then convert the %/em value into pixels? I am doing
image replacement (byt getting the text and passing it onto a server
side image generation web handler) and as a result the font size is
76px rather than 24px (due to using var fontSize =
parseInt($this.css("font-size")))