Which web browsers / versions support document.activeElement ?
document.activeElement
This property allows you to see which element is active / has focus.
Are there any major errors / differences between implementations?
document.activeElement is supported by IE6 +, FF3 +, Safari 4+, Opera 9+, Chrome 9+. (FF2, Saf3 do not support this property)
It still does not work in WebKit-based browsers on the xhtml page, see https://bugs.webkit.org/show_bug.cgi?id=63922