Is it? I haven't had much time to check out the other entries but I would have thought Twine would be a popular choice for a jam entirely focused on browser games.
I'm familiar with how to style Twine games using CSS but didn't get a chance for this one: I finished within an hour of the deadline and felt as though the default white text on black was preferable to something cobbled together at the last minute. I'll have to look into CSS variables, though: if it offers a way to adjust the style throughout the game then that could be a lot of fun. Thanks for the links!