If you code websites for handheld devices such as mobile phones here is a pretty interesting online validator for your pages.

Wait, there is more! Here are three very useful sites for mobile websites:

http://msdn2.microsoft.com/en-gb/library/aa455233.aspx
This is an overview of the Properties that are available in Cascading Style Sheets. These properties can be spatial, related to an aspect of the document, controlling layout or other such aspects of page design.

http://msdn2.microsoft.com/en-gb/library/aa455315.aspx
Internet Explorer Mobile supports the following Wireless Markup Language elements.

http://msdn2.microsoft.com/en-gb/library/aa455316.aspx
There are many elements in the XHTML language and many derivatives. This topic is designed to help understand which elements are supported in which derivative and within Internet Explorer Mobile.