CSS Grid and Flexbox not working when overlay is running

[Barsnes]

I have created a new custom hud, using CSS properties such as grid and flexbox, but these seem to not be supported when running the overlay.

It works perfectly in the browser:

But once I try to run the overlay, everything seems to be messed up:

I am suspecting this might have to do with what "browser" is being used to run the overlay?
Here is my current hud and code:
https://drive.google.com/open?id=1Crmx8rim4Mt6p_W6T0SHLZwj8hHFLUbQ