The Birth of XHTML
The success of XML rather left HTML in the lurch because HTML documents are not valid XML documents the new XML processor cannot read them. On the other hand HTML browsers cannot read XML documents but there was a growing need to integrate some of the newer languages, such as MathML [Mathematical Mark-up Language] into HTML documents. So XHTML was born.
XHTML is an XML-based language that reproduces the functionality and structural rules of HTML, but subject to the rules of XML, and uses the same mark-up elements names. The main differences are:
All elements and attribute names must be in lowercase.
All empty elements must be written using XML's special empty-element tag.
All end tags must be present and there are no optional end tags.
The HTML elements must contain a single head element, followed by a single body element or a single frameset element.
Every head must contain a single title.
Many of these terms will become clearer in the next section