The Opera browser (Opera 7+) supports the same two rendering modes (Quirks and Standards) as Internet Explorer (for details, see http://www.opera.com/docs/specs/doctype/).

Mozilla Firefox 1+ supports three rendering modes: Quirks mode, Almost Standards mode, and Standards mode. Firefox’s Almost Standards mode corresponds to Internet Explorer’s and Opera’s Standards mode. When a page contains a valid XHTML 1.0 Transitional DOCTYPE (and it is served with a text/html MIME type), Firefox renders the page in Almost Standards mode. When a page contains either an XHTML 1.0 Strict or XHTML 1.1 DOCTYPE (or the page is served with an XML MIME type), the page is rendered in Standards mode (for details, see http://www.mozilla.org/docs/web-developer/quirks/doctypes.html).

You can determine a browser’s current rendering mode by temporarily adding the following client-side script to a page (this script works in the latest versions of Internet Explorer, Firefox, and Opera).

<script type="text/javascript">
    alert( document.compatMode );
</script>

You need to care about the browser rendering mode, because it affects the way in which Cascading Style Sheets are applied to the page. If you convert your existing HTML pages into XHTML pages, they might look very different when you open them in your browser.

For example, Internet Explorer calculates the size of page elements in different ways, depending on the rendering mode (it uses a different CSS Box Model). In Quirks mode, the width of an element is calculated by summing the width of the element’s content, padding, borders, and margins. In Standards mode, the width of an element is calculated by taking into account only the width of the element’s content.

For example, consider the following two tags.

<div style="width:400px;border:solid 1px black">
First Box
</div>

<div style="width:400px;border:solid 1px black;padding:10px">
Second Box
</div>

The two elements are the same, except for the second element’s additional padding. In Quirks mode (see Figure 2), the two elements appear to be the same size, because the additional padding of the second element is taken into account when calculating its width (the total width of both elements is 400px). In Standards mode (see Figure 3), the second element appears wider than the first element, because padding is not taken into account when calculating the width of an element (the total width of both elements is wider than 400px).

Figure 2. Quirks mode

Figure 3. Standards mode

This is only one example of browser differences in Quirks mode. In Quirks mode, each browser implements the W3C Cascading Style Sheet standards in significantly different ways. The beautiful thing about switching to Standards mode is that it forces almost all modern browsers to interpret the W3C standards in a very similar way (not exactly the same, but much better).

If you want your Web pages to appear in the same way across browsers, then it is a good idea to trigger Standards mode (in Internet Explorer and Opera) and Almost Standards mode (in Firefox), by including an XHTML 1.0 Transitional DOCTYPE.

对于使用了XHTML而导致document.write不工作的原因，我翻译了Ian Hickson的一篇文章：为什么document.write在XML中不工作 http://neoone.51.net/tech/dw_not_work.php

有一些用户需要在XHTML中用到Google AdSense，也就是需要使用document.write，这里是一个基于PHP的解决方案：使Google广告AdSense在XHTML中工作 http://neoone.51.net/tech/adsense.php

为不同的浏览器使用不同的MIME Type：使用正确的MIME类型伺服XHTML http://neoone.51.net/tech/mime_type.php