You should read the rest of Henri’s article, because I’m simplifying immensely here. Even in IE5/Mac, there were a few older doctypes that didn’t count as far as opting into standards support. Over time, the list of quirks grew, and so did the list of doctypes that triggered “quirks mode.” The last time I tried to count, there were 15 doctypes that triggered “standards mode,” 5 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.
— Mark Pilgrim