Web pages are written in a language called Hyper Text Markup Language; HTML
The latest version of HTML is 5.
There is a move to upgrade the code to Extensible Hyper Text Markup Language.
XHTML is compatible with XML (EXtensible Markup Language) allowing applications to more easily exchange data.
In preparation for making the change, edit your web pages so that element and attribute names are in lower case. XHTML is case-sensitive.
Use <p> rather than <P>.
Also end tags are required. Make sure that <p> is followed by </p>.
<br> should be written <br /> and <hr> as <hr />.
For more information see:
The W3C validation site
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