Just curious - With modern browsers zooming the whole page, rather than simply increasing the font size as in the past, is it still as important to use em/%/etc rather than px for setting font size?
Since zoom isn't the only option (there's also the much-more-awesome text-enlarge, which makes text readable for me without distorting images or causing sideways scrolling like zoom does), you'll have to take into account that IE (even IE8 and 9) do not increase text size if you set them in pixels.
But beyond that, I dunno. Since I hate zoom and love text-enlarge, it might be just me. And I used to think Safari didn't have zoom but apparently they've added it now.
I also like the idea that a user agent such as one on a mobile can increase text without using zoom (since zooming on a web site is sucky usability).
I think some browsers won't zoom text whose size is in pixels.
I prefer to use em's - but that is just me.
With browsers having a zoom feature, I don't bother with text enlarging scripts anymore. The zoom feature works quite well imho.
It is still important to avoid using px to size text, because there are still plenty of people using browsers that don't have page zoom, and there are also plenty of people who prefer to change the size of the text rather than zoom the whole page (which invariably makes a mess of images and often doesn't do great things for the layout).
There's no reason to set the size in pixels rather than %/em, and good reasons not to.
I've not come across that one. Page zoom should enlarge everything on the page, including images (although it doesn't tend to work for other embedded objects) and layout, so certainly should include fonts sized in pixels. IE8 definitely does zoom pixel-sized text. Of course, you've still got to cater for browsers from before the days of page zoom.
With browsers having a zoom feature, I don't bother with text enlarging scripts anymore.
I've never seen the point of replicating any browser functionality with fragile and restrictive widgets in the web page - not for printing, not for going back a page, and certainly not for changing the size of the text. Show people how to use their browser properly, rather than giving them props that stop them from learning.