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