I just checked the source of the OP linked page, and this link is not in the page anywhere.
Likely an unintentional omission!(?)
I'd suggest callouts in <h1>s myself, ideally at the start and the end of the article.
--
As an aside, it's... reasonably fast, but not playable. And all the monsters appear to have quietly been turned off, and because the game loop can't see your exact keystrokes you can't cheatcode them back on :P
I think any potential customers would understand that scaling isn't the point of the demo. It's unreasonable to assume that they'd infinitely scale a processing- and bandwidth-hog like this just for fun.
This runs super well for me.
Looking at chrome developer tools it's sending out a new frame request about every 38ms.
Runs much nicer than the top two google results for "play doom online".