.html or .txt - a worksheet text file with html code and cells surrounded by {{{ and }}} that defines a worksheet
.html - the URL of a page of the Sage Documentation, for example the URL of this page
.html - an HTML page generated by docutils from a ReStructuredText file
.rst - a ReStructuredText file (Add ".. escape-backslashes" as a line in the file if you want backslashes to be escaped. Put latex code inside backticks "`" or dollar signs "$" or "$$". Math role is not supported for now.)