Xapti - 12:44 am on Aug 28, 2011 (gmt 0)
While people already mentioned that one should use EMs or % for font sizes, Another important factor when doing this is allowing the website to flow.
Elements which do not have automatic width (generally should be all but one along any horizontal point) should be sized with EMs (not % since it is interpreted differently for containers) as well, so that the containers scale properly and do not break the layout.
The biggest problem a designer may encounter is dealing with images. They should not be a major issue when you design properly though. In cases where an image should fit into a certain text area (mixed content), it is perfectly valid to also scale the image with EMs (yes you can do that — use style="" or a style sheet though). It may not appear the best depending on the browser's anti-aliasing/interpolation, but generally it will look great nowadays.
All too commonly, I see websites which have constricted containers resulting in broken-looking pages, I think it's terrible.