EM units with CSS have always escaped me. It is a much better practice to use these units to scale text rather than pixels, since they are more flexible when users what to increase the font size. However not long after starting I always find problems and fallen back to pixels once my head started hurting.
I found this article gives a clear guide on how various EMs should/can be used to control font-size with css. How to convert back to pixels, and what to do with the problem of having nested elements.
On a slightly related note, I found a great program, "Accessible Form Creator" from HiSoftware. It allows the easy creation of accessible forms. Just enter the details for each field and it will create the all the tags automatically. Much easier than remembering all the attributes to follow current guidelines.
Only criticism I have of the program is that it would be nice to be able to tailor the tags used surrounding each input field. I prefer to use paragraph tags to wrap around each of my form elements, rather than the divisions that the program uses.
Still it is sure to take the pain out of typing up forms again and again.