I don't test "personal" code on IE9. I couldn't if I wanted to: the only Windows license I have is for XP which I have running on my main desktop. At some point I might upgrade to 7 (unless before then I decide to live without games and switch to Linux on the desktop instead of moving to another Windows version) but that won't be until I next rebuild that machine. All my other computing resources run Linux in some form or another.

Perhaps the author of this code is in a similar position, and has no Windows Vista/7 installs to test via IE9 with? It doesn't matter how decent IE9 is, if it won't run on your current platforms.

Testing on Chrome, Firefox and Opera is more universally easy as their latest desktop releases will run on Linux, OSX and XP+.

If a missing or malformed DOCTYPE is an issue then that is not IE9's fault, but you can't criticise the author for not explicitly testing on IE9 if he/she can't do so.