One of the first things you learn about XHTML is lowercase. Everything is lowercase. Lowercase tags, lowercase attributes, lowercase, lowercase, lowercase…
Thankfully, there’s a fix. All it requires is to edit the hundreds of entries in the tag library. Doing so through the interface tag library dialog is painful enough to make you want to claw your eyes out. So to save you the insanity, I did some file trickery and fixed all of the event entries within the entire HTML tag library.
Simply download the HTML Tag Library and overwrite your existing one. You’ll find it by default in “C:\Program Files\Adobe\Adobe Dreamweaver CS4\configuration\TagLibraries” on x86 systems and “C:\Program Files (x86)\Adobe\Adobe Dreamweaver CS4\configuration\TagLibraries” on x64 systems.
Update: If you are using CS5, you will want to download the new tag library.