For More information, please get to here: Document Object Model Core
Some extra information about IE6 and IE8(from wiki):
Internet Explorer 6
This version included DHTML enhancements, content restricted inline frames, and partial support of CSS level 1, DOM level 1, and SMIL 2.0. The MSXML engine was also updated to version 3.0.
Internet Explorer 8
partially supports CSS Level 2 and DOM Level 2, with major implementation gaps and conformance issues. Almost full conformance to CSS 2.1 has been added in the Internet Explorer 8 release.