The "/builtin" page:

Return one of many built-in content files. Query parameters:

name=FILENAME

Return the single file whose name is FILENAME.

mimetype=TYPE

Override the mimetype in the returned file to be TYPE. If this query parameter is omitted (the usual case) then the mimetype is inferred from the suffix on FILENAME

m=IDLIST

IDLIST is a comma-separated list of integers that specify multiple javascript files to be concatenated and returned all at once.

id=UNIQUEID

Version number of the "builtin" files. Used for cache control only.

At least one of the name= or m= query parameters must be present.

If the id= query parameter is present, then Fossil assumes that the result is immutable and sets a very large cache retention time (1 year).