Can't you use tampermonkey or a similar tool that lets you apply your own stylesheet?
Things like colors, contrast levels, font sizes, are often matters of personal preference, and the browser (in theory) is the common place to manage those. Each site should not have to reinvent this feature.
This would send me into peals of laughter if I weren't already crying. The time to make this argument was 30 years ago, when the web wasn't fragmenting into a billion different pieces. The browser can make exactly zero assumptions about any given site, so it could never be a place where user preferences about them could be actionable. Downvoters should get work as web developers sometime. You really want the browser making assumptions about your web design?
All it can do is pass a header and let the website do what it will with it.
A user's custom style-sheet might be good for one web-site, but not for every website.
The original web was much about self-expression of developer-users, but now the web is all about apps, which must not break because a user might want to use different colors.
And why should you need to customize colors? I can understand that different users need larger fonts which you can do by zooming in the browser. Colors should be good to go if the website is well-styled to begin with.
What if it is not well-styled? Or, maybe some people think it is and others disagree and want something else. The end user should need to customize fonts (not only larger, but also if you want smaller fonts; I more often find the fonts on a web page are too big and want smaller fonts; however, also you might prefer a specific font typeface and not only the font size), colours (also for many reasons, including using a monochrome display or printer that the web page author might not have been aware of), animations (e.g. to disable them), margins, etc.
Pretty sure AI-driven style derivation will finally deliver the dream of custom stylesheets in a robust and automatic way.
That works today. No problem.
[role="button"] {
cursor: pointer;
}
Not sure what you mean. Between the new selector combinators and attribute selectors, you can do a ton. You also have style-based container queries, which are probably close to what you want.