HTML and XHTML are very similar syntactically. There are some fundamental things that make both very different however.
The most important difference between the two versions of HTML is the strictness. Simply put, a standard web page written in HTML is not strict, whereas one written in the XHTML serialisation of HTML is strict. Here is a list of things that XHTML brings to HTML.
- A DOCTYPE is an essential
- All tags must be closed. This means things like breaks and images must be closed.
- All elements and attributes must be lowercase
- Elements must be properly nested
- There must be exactly one root element
- Attribute values must be quoted
Some people would say that XHTML is easier to learn than
standard HTML, due to the strictness. This means that
<h1>Page<p> cannot be included,
for instance. In HTML this will run
correctly, but takes longer to parse and therefore is not fully
qualified HTML and not following the validation rules for
well-formed HTML. One should be careful with such HTML, as it
can display differently as per browser and platform. As such,
HTML which does not follow the W3 recommended rules should be
This is where the strictness of XHTML comes in handy. The XHTML would have problems altogether and would not display a page which cannot render correctly. Some also say that HTML5 is more strict than previous versions, and there is no current XHTML5 specification that has been standardised.
The argument about why to bother with XHTML is becoming less and less important over time as HTML5 progresses and on its own becomes more strict.
As XHTML takes a long time to become a standard after a version of HTML is released, it tends to be ignored by early adopters of the latest HTML standards.