https://github.com/Kodiqi/WindowFocusThank you @PelicanPolice!
That information pointed me to a direction. I can confirm that I have the same issue on my Linux machine an Chrome, however things work fine on my Win8.1 machine.
The good news is, I found some sources that point to causes and solutions.
Basically, some say it's something YoYo Games did in their patch sometime since November 2020. Someone mentioned that a workaround is to programmatically set window focus somewhere somehow someway. I will investigate and if it works, I will spread the news. :)
https://www.reddit.com/r/gamemaker/comments/kfs5cs/keyboard_inputs_no_longer_wor...
https://www.reddit.com/r/gamemaker/comments/lcbyii/html5_game_embedded_in_browse...