For html_begin//1, Env is a term Env(Attributes); for
html_end//1 it is the plain environment name. Used for
exceptional cases. Normal applications use html//1. The
following two fragments are identical, where we prefer the first
as it is more concise and less error-prone.
html(table(border=1, \table_content))
html_begin(table(border=1)
table_content,
html_end(table)