The last time I tried to count, there were 5 doctypes that triggered “almost standards mode,” and 73 that triggered “quirks mode.” But I probably missed some, and I’m not even going to talk about the crazy shit that Internet Explorer 8 does to switch between its four — four! — different rendering modes. Here’s a flowchart. Kill it. Kill it with fire.
And just in the flowchart link will break anytime soon, here is a copy of the image.