I had a related thread about converting a site from px to em. The link is at the bottom of this page.
Now I think I've had a reasonable outcome with my px to em conversion but there is a problem.
When you zoom in the browser beyond the limits of the screen some of the content gets cut off by the browser window. It's obvious when you scroll right after zooming.
I think that it must be to do with overflow:hidden.
At the bottom of the page is a row of thumbs that have a mouseover image swap. I removed overflow: hidden from that containter and the rollover stopped working and the space between the alternative navigation at the very bottom of the page closed up.
I would just like to know a bit more about what's going on and the relationship between overflow: hidden and browser zoom.