How to insert CSS code into a page if I can't insert it into head tag? Only inline mode?

How to insert CSS code into a page if I can't insert it into head tag?
Only inline mode?

I have to insert some CSS style into a Plain HTML module inside a CMS. The
problem is that I have no access to the template so I can't insert the CSS
into a .css style file.
I had thought that I could use the tag (that would enable me to insert the
CSS styles directly in the HTML) but reading online seems to me that I can
only insert it into the tag and I can't do it because I have not access to
the template code.
So my questions are:
Can I use the tag outside of the tag?
If not the only solution is to use inline CSS or can I try something else?
Tnx
Andrea