I think we really need to be able to more cleanly separate content from the design, and let the browsers, a.k.a. user-agents, decide/override more. Browsers have ceded too much control over layout and visual structure to web designers, who go on to make (often incorrect) assumptions about how the user wants to interact with their content.

the problem is that there is no distinction between a webapp and a document, the line is thoroughly blurred. For documents, you can probably just override a fair bit of schenanigans and things will still work, if look somewhat ugly, but for a spreadsheet app that's a rube goldberg machine of javascript and continuously regenerated html and css, it'll just turn it into a broken mess. the browser has become the de facto cross compatible application platform, which requires handing a lot of control to websites.

That sounds similar to a CSS proposal called container queries, which has been a much-requested feature for many years. Currently the only native way to do responsive design is to make styling changes based on attributes of the viewport (like width). But like you say, it would be nice to have dynamic layouts for every individual component of a website. Container queries do that.

yeah I heard of those, I was blanking on the name, thanks for pointing that out. Another thing I really want would be position:device-fixed; but that one doesn't even seem to really be on the table for the most part.