You can modify the styling of elements in Javascript with modern browsers, but I'm not sure it is exactly "standard", in the way that CSS is "standard". A modern form of JSSS might look like this: an HTML document with no styling/CSS, but some JS that styles each element when it is loaded. This might cause some flicker [0], because the HTML would be rendered un-styled and then re-styled by your JS code. You can mask that flicker with a sort of "loading screen" -- oh, now I know why I see those tacky pre-loaders on so many websites.
The Internet is a flat circle, in which old technology is constantly re-invented by semi-literate people. This will surely be the sole Horseman of the Internet Apocalypse: a black-and-purple checkered horse, the rider wearing a hoodie that says "my prompts bring all the ladies to the yard" and wielding AR goggles. He speaks a dozen words to command his AI legions -- they do his bidding in a thousand-thousand jobs: getting hired, attending meetings, writings emails, getting fired; each in a cycle fast enough that he is ever employed, ever influencing, ever disrupting. We have assuredly earned all that he does to the Internet.
[0] = This is alluded to in TFA: "You'll also want to be careful with your style-setting functions: get too complex and it will take your document forever to render itself in the user's browser."
