I'm getting the error listed when I use the Itch.io dektop app, have the game downloaded, and click the Launch button. Since for several "Play in Browser" HTML games, the Itch launcher can create a standalone HTML window all by itself. Not in a web browser.
The issue with web browsers is, as many have stated, the keybind for Chrome to close the tab you're looking at, fullscreen or not, is the same as sneaking forward (Ctrl + W). https://support.google.com/chrome/answer/157179?hl=en&co=GENIE.Platform%3DDeskto...
Tho, at least it seems Firefox doesn't use the same shortcut, all that Ctrl+W is listed to do there is close the current file when using the JavaScript Debugger. So, I'll probably have to use that instead.