You can write
in a topic or template. This variable accepts the following parameters:
_DEFAULT optional, id of the head block. Used to generate a comment in the output HTML.
text optional, text to use for the head block. Mutually exclusive with
topic optional, full Foswiki path name of a topic that contains the full text to use for the head block. Mutually exclusive with
requires optional, comma-separated list of id's of other head blocks this one depends on.
expands in-place to the empty string, unless there is an error in which case the variable expands to an error string.
to generate the sorted head tags.