XHTML documents are backward compatible with older, non-XHTML compliant web browsers. Instead of sloppy HTML tags, your pages will now contain XML tags that are always properly closed and nested correctly, such as:
<p><b><u>This is some text<br /><br /></u></b></p>
<p><b><u>This is some text<br><br></b></u>
You can see that the XHTML version of the code above has a <p>, <b>, and then a <u> tag. These tags are closed in the reverse order that they were created in: </u>, </b>, and lastly </p>. All tags must be closed in this way for the XHTML document to be considered valid.
Also, notice the <br /> tags? Because of the way non-XHTML browsers are designed, as long as you leave a space between the beginning of the tag and the “/>”, then they will just treat the tag normally, and ignore the forward-slash.
XHTML is a standard
XHTML is an accepted standard (see http://www.w3.org/TR/xhtml1/), meaning that all of the newer browsers due for release in the future (such as newer version of IE, Netscape and Opera) will most definitely contain built-in support for XHTML.
The first document type in the XHTML family is XHTML 1.0. The W3C standard for XHTML takes three of the previous document types from HTML 4.0 and converts them to fully utilize XML wherever possible. This promotes a consistent, logical layout, while still keeping the actual content easy to follow.
XHTML documents must incorporate one of three document type definitions (DTD’s). This makes sure that the XML data contained within an XHTML document is valid and conforms to a certain layout/logical style as defined in that DTD file (more on DTD’s below).
You might be wondering what the benefits of migrating your current HTML documents over to XHTML are; you may also be wondering why you should bother learning more about XHTML; well, let me point out some of the benefits of XHTML (as listed at http://www.w3.org/TR/xhtml1/):