Which is the easiest way of making an .html file with Trisquel Belenos?

I'm not sure I understand. You can write an html file in any text editor and save it as 'file.html'. Are you looking for something that will generate a template?

I use SciTE.

sudo apt install scite

LibreOffice can read/write HTML.

In the O.S I have been using during these last 9 years it was an option of one of the browsers I could use.
But I can't find something similar here.
I gather your OS of nine years was Puppy.

Just curious, what exactly is the "missing" feature and the browser you can't find in Trisquel?

Pale Moon, Developer Tools, Geany, text tags?

@ mason, loldier, Magic Banana
Thank you very much.

The data and questions with which you have contributed, have encouraged me to deepen my studies on .html editing.
Considering the set of options I already knew and the new ones that you have suggested, I think, that for the time being, the best substitutes for my Seamonkey dependent Compose is a combination of LibreOffice Writter and Abrowser's Web developer tools.

Thank you all for trying to help