I don't like using Browsers for Construct 3, I only use the AMAZING Desktop version, so it may work differnt on chrome or whatever browser you're using I guess.
Anyway, this is what I do to make Full Screen Stretch as soon as the game starts, give it a try:
I created a boolean variable: "FullScreen" set it's initial to TRUE.
System On Start of layout |
(sub-event) System iS FullScreen | Browser Request fullscreen - Stretch (letterbox scale)
Not sure if that's what you're looking for but it Works for me.