It is true that trying to make a website Javascript and CSS free is becoming a very limited proposition. The problem with pure HTML sites using SSR is that they have to assume the user-agent defines some sort of contract around features when absolutely none has ever existed. And browsers have to assume SSR sites will update behavior correctly when the user agent string changes, which also has seldom been the case. This is why all user agents still include Navigator, Gecko, KTML, WebKit, etc in their strings for regex to chew on.
The latest iPadOS, for example, purposely reports itself as a Mac desktop because too many sites saw 'MobileSafari' and returned a sub-par site experience based on a small mobile phone screen. But it actually goes beyond that:
1. It lets the user toggle the user agent themselves between mobile and desktop (iPhone and Mac Desktop style agents)
2. The browser will by default use a mobile user agent when rendering in a small form, such as a browser in a smaller modal window or a side-bar.
Client hints might provide a way to actually formalize a contract for SSR apps, if people participate and indicate what they need.
The latest iPadOS, for example, purposely reports itself as a Mac desktop because too many sites saw 'MobileSafari' and returned a sub-par site experience based on a small mobile phone screen. But it actually goes beyond that:
1. It lets the user toggle the user agent themselves between mobile and desktop (iPhone and Mac Desktop style agents)
2. The browser will by default use a mobile user agent when rendering in a small form, such as a browser in a smaller modal window or a side-bar.
Client hints might provide a way to actually formalize a contract for SSR apps, if people participate and indicate what they need.