Any libre, DRM-free, web-based, HTML5 game that you can recommend me?

>I've tried minetest (with mobs mods) but I do not have anyone else to play it with

Well, you can join #lgn on freenode. I'm sure you'll find a mate or two who'll enjoy playing it with you.

http://browserquest.mozilla.org I believe ... although LibreJS does not recognize the Javascript as free.

The only notable libre JavaScript game I can think of is Freeciv-web, but I wouldn't recommend playing Web-based games in the way they are usually played. You should download and keep your own copy that you run, and if you do that, there's not much benefit to using the JavaScript version. See here for why that's my recommendation:

https://onpon4.github.io/other/kill-js/

As for games in general, these are some of the ones I personally can recommend in no particular order:

- Naev
- ReTux
- Endgame: Singularity
- MegaGlest
- Warzone 2100
- SuperTuxKart
- Freeciv
- Flight of the Amazon Queen
- Project: Starfighter
- Xonotic

Just off the top of my head.

There are a number of web based games.

HexGL is a nice example IMO.
http://hexgl.bkcore.com/

I believe Ancient Beast also falls under your list of requirements.
http://ancientbeast.com/

"Or also any good game for PC, I've tried minetest (with mobs mods) but I do not have anyone else to play it with."
I literally don't know where to begin. I love playing games. If you are looking to play games with people you should check out the Libre Game Night.
http://libregamenight.xyz/ Check the IRC cahnnel, you can probably find someone there willing to network play most days of the week.