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Perform the meaning of this tag.
This is defined by the tag, for example the bold tag <B> may switch
bold text on and off.
Only a few tags have semantic meaning to the parser. These have to do
with the character set to use (<META>) and the base URL to use
(<BASE>). Other than that, the semantic meaning is up to the
application and it's custom nodes.
The semantic action is performed when the node has been parsed. For
composite nodes (those that contain other nodes), the children will have
already been parsed and will be available via
#getChildren.