This well-formed and valid XHTML document has a style body element. The body's background is set to a light yellow and its margin is set to 20 pixels. When the document is sent as text/html, you see the background color across the entire page, but when it's sent as application/xhtml+xml, the background should only cover the actual dimensions of the body element. This is because the CSS specification has certain provisions that only apply to HTML documents and not XML document like XHTML. (Note that Opera 8.5 handles this test incorrectly, although this will likely change in a future version.) The DOM specification also has some similar provisions.