An intro container extension. If the content attribute is defined, then it is
assumed that we have XHTML content in an external file. Load content from
external DOM. No need to worry about caching here because this is a transient
model class. It is used and then disregarded from the model.
Just like in a page, the styles and altStyles strings can be a comma
separated list of styles. Handle this by storing styles just like pages.
Returns the elements loaded from the content attribute. This is the content
that should be inserted for the extension. If it is a file, all child elements
of body are returned. If it is a file with an id, only the element with the id
is returned.
the elements to be inserted
