Just because a font has "loaded" does not mean it's ready to be used. This very problem makes it hard to load @font-face fonts on demand in <canvas> apps as the

Just because a font has "loaded" does not mean it's ready to be used. This very problem makes it hard to load @font-face fonts on demand in <canvas> apps as there is a certain amount of time you must wait for the font to actually load that is not exposed or accounted for in any browser APIs. Even if you include a font in @font-face with a data: URI, you cannot immediately start using it.