Yes I have done changes in several files, but last time I tried to use a childtheme — the stylesheet didn't read for some reason. I might have forgot to link the new css-file, but I don't know if it supposed to be needed if I use all codes in the childs css?
I'm familiar with W3 validators since years back! I don't pay much attention to it, neither Google does I suppose?! I just thought that those errors had an impact of using em as you mentioned it :)
For now I just want to stuck with themes like Twenty Ten or Eleven, since I find them to be most compatible with many plugins I use etc.