@leocg I have no clue how to use those geeky stuff, but I found it, I had to install Opera Developer though...
Will this come to the Opera Browser in a new update maybe?

I'm keeping my Spotify web player in a tab pinned all day long. Most probably many other users are listening to music through a website while browsing (Spotify, YouTube etc.)

It would be awesome if you added a controller for these sites, or maybe a sidebar option. I would love to be able to control Spotify through the sidebar.