I have this weird bug where the interface of our game only shows up, when the user clicks  the blue "fullscreen" button. Otherwise, when simply playing in embed mode, the game interface will not show.

Any idea why? The WebGL build settings in the Editor maybe?

Here is the link:   https://mathiasahrens703.itch.io/a-taxi-for-billy

Thanks for your help!

Same for me... Just write a big text on the description that says to use fullscreen 

Sounds good, thanks RemKed

I ended up finding a solution by editing the canvas settings in the Unity Editor : from Constant Pixel Size to Scale with Screen Size

After that, you must edit your resolution in player settings and  the itch.io dashboard embedded manual resolution to 960 x 590

This video shows how to do it. I just tested it and uploaded a new WebGL version, and it fixed my interface issue :)