When SGML, forces the content-type to be text/html.
When XML, forces the content-type to be application/xhtml+xml.
When auto (the default), the following steps are performed to resolve the serialization:
Use the serialization active on the current request.
Use the application context-paramcom.aoindustries.encoding.Serialization.default
that allows an application to control the selection of HTML versus XHTML.
When this context-param is unspecified or auto,
determine the serialization based on the client's Accept header.
This should normally be left with the default value of auto. This is used
when the content is known to contain invalid markup that can not be parsed as XML.