I was getting the jitter pretty quickly once enemies started showing up, and it stuck around even with no enemies on screen. I did a bit more testing and it looks like it does not do it at the default embedded size, or 1080p fullscreen, but only in 4K fullscreen. With the pixel art graphics I had not thought the 4K would be an issue and wrongly assumed it to be a design choice, but it looks like it was the resolution. Moving it to a 1080p monitor it runs fine. We play and review jam games as a family, so put them on a 4K screen to let everyone see them better.
We primarily use the Vivaldi browser (Chromium based). I've now also tried it with Firefox, and it seemed better, though still had a bit of jitter. We are also on Linux, so that may have an effect, though tends not to be an issue with most web builds.
I tried again to get to the Credits. It highlights in yellow, and when I click it goes down and turns white. It stays that way even as I mouse over the others and back over it. If I press the up arrow key it will go back to yellow when the mouse hovers over it. No other buttons I tried got it to change.