Validate as XHTML
From b2evolution manual
This manual page is outdated. NEW PAGE: Validate as XHTML in b2evolution v5+.
 What is XHTML?
XHTML is the next generation of HTML and is a hybrid between HTML and XML. XML was designed to describe data. HTML was designed to display data. XHTML is much stricter than HTML. Not all browsers support XML so XHTML provides an intermediary solution and can be interpreted by XML and HTML browsers. For further information see: http://www.w3c.org/MarkUp/.
 Why should I care?
In b2evolution, all of the evoskins (the design layouts you can choose for your blog) are written to satisfy the XHTML standards. This ensures that the webpages produced by b2evolution are properly rendered by most all browsers currently in use. This will also ensure that the webpages produced by b2evolution will be compliant with web browsers far into the future.
 How can I test my site to see if it validates as compliant?
On most b2evolution skins there will be a validation link to check your site (usually at the bottom of your blog or at the bottom of the sidebar). Simply click it and your site will be tested by the W3C Markup Validator (direct link). After the test runs it will tell you whether or not your webpage is compliant or not. If your site does not pass the test the validator will point out the errors that it found which you can then go about correcting.
You can search the internet for many sites which explain the differences between HTML and XHTML markup and how to make sure your code is valid.