I think here you should be use IE Hacks and you can also read about other hacks chrome
and firefox to be able check versions and match them, but i wanna focus on specifiec browser - Internet Explorer
So as i started to say, you can use IE hacks to match fontsize, example:
<!--[if IE]>
<style>
span{
font-size: 30px;
}
</style>
<![endif]-->
<style>
span{
font-size: 20px;
}
</style>
Like I said, you can use firefox/chrome and other browsers css "hacks" to check version, and check supporting @fontface
, I've not gave you fontface example since I don't have link and i perfer not make a confuse.
Hope I helped.
EDIT: I'm not sure if I'm clear then I would like make a fast point:
You can check each browsers (also each version) and see if he support fontface, if he doesn't you just use the same browser hack to change span font-size.