Do modern browsers care about the DOCTYPE?

If you use deprecated attributes or tags <center>, <font color="red">, or <td valign="top"> etc. in XHTML 1.0 Strict (no depr. attributes), modern browsers (I will use Chrome as an example) still take notice of and use them.

If you use HTML5 <video> on an XHTML 1.0 Strict DOCTYPE Chrome will still recognize it - it's not as if they'd program it to not. I tested the worst deprecated, capitalized, and unquoted attribute code I could write, along with HTML5 audio, with the XHTML 1.0 Strict DOCTYPE in Chrome and it rendered flawlessly.

Here's the code I tested, working flawlessly in Chrome (red bg, centered table, audio playing):

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
   "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Do browsers care about the DOCTYPE?</title>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8" >
</head>
<body bgcolor=#ff0000>
 <CENTER>
  <table cellpadding="0" cellspacing=0>
   <tr><td valign=top>test</td></tr>
  </table>
 </CENTER>
 and some HTML5 audio..
 <audio autoplay>
  <source src="http://www.duncannz.com/resources/sound/alarm.mp3" type="audio/mp3">fallback text</audio>
</body>
</html>

So my question: Do modern browsers (translation: browsers other than IE) pay any attention at all, or do anything differently, because of the DOCTYPE? Do they even bother to read and interpret it?

Solution

Browsers do care about the DOCTYPE - otherwise there wouldn't be any point in having it!

You are right in saying that many browsers interpret old/deprecated commands in the correct way, but this is largely a matter of backwards compatibility. There is such a huge amount of content on the web that it is next to impossible to keep everything up-to-date and standards-complient. The web browsers continue to support these outdated pages because if they didn't, much of the content on the web would look slightly off. Most users don't know the difference between HTML4 and 5, so the blame could fall on the browser, which could be devastating - especially if a page looks bad on Firefox and nice on IE!

The DOCTYPE is mainly used in validation and to determine whether a browser runs in this "quirks mode" (where many of these older rules still work) or "standards mode" . Many professional web designers use the W3C validation tools to make sure their web pages are valid HTML, and the tools provided by their website look at the DOCTYPE to choose the correct set of rules with which to validate. Furthermore, XHTML strict does not allow empty tags or other blatant syntactic errors.

Hope this helps!