The Doctype

From the top:

<!DOCTYPE html
          PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
          "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

This is called the doctype. There’s a long history—and a black art—behind the doctype. During the development of Internet Explorer 5 for Mac, Microsoft found itself with a surprising problem. The upcoming version of its browser had improved its standards support so much, older pages no longer rendered properly. Or rather, they rendered properly (according to specifications), but people expected them to render improperly. The pages themselves had been authored based on the quirks of the dominant browsers of the day, primarily Netscape 4 and Internet Explorer 4. IE5/Mac was so advanced, it actually broke the Web.

Microsoft came up with a novel solution. Before rendering a page, IE5/Mac looked at the “doctype,” which is typically the first line of the HTML source (even before the <html> element). Older pages (that relied on the rendering quirks of older browsers) generally didn’t have a doctype at all. IE5/Mac rendered these pages like older browsers did. In order to “activate” the new standards support, web page authors had to opt in by supplying the right doctype before the <html> element.

This idea spread like wildfire, and soon all major browsers had two modes: “quirks mode” and “standards mode.” Of course, this being the Web, things quickly got out of hand. When Mozilla tried to ship Version 1.1 of its browser, it discovered ...

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