def placeholder(self):
return '<STR_LIT>'<EOL>
Returns the default, empty output value for the renderer.

        All renderer methods use the '+=' operator to append to this value.
        Default is a string so rendering HTML can build up a result string with
        the rendered Markdown.

        Can be overridden by Renderer subclasses to be types like an empty
        list, allowing the renderer to create a tree-like structure to
        represent the document (which can then be reprocessed later into a
        separate format like docx or pdf).