XHTML or Extensible HyperText Markup Language is the successor to HTML. It's a reformulation
of HTML using XML and is therefore almost identical. However, XHTML is stricter so there
are a few differences that we need to look at.
First, XHTML requires all elements to be closed. So previous optional end tags must be included
and elements that does not have an end tag gets one in the start tag by adding a forward
slash.
Secondly, XHTML is lowercase-only for all tag and attribute names.
And all attributes must have a value specified. For attributes that don't have any values
this means that they must use their own names as their value. Also, double quotes are required
around all values.
It's recommended that we specify an XML namespace using the xmlns attribute. By placing this
attribute within the tag we decide the namespace used throughout the document.
This attribute can be placed within any element to define one or more alternative namespaces
within our document.
There is also an optional line that we can place at the first line of the document. This
XML directive tells the browser that we are using the current version 1.0 of XML along
with the common 8-bit Unicode character set.