You can just load that content in another browser.
If your browser handles all of the popular sites, people will use it. Nobody cares about whether Flash loads anymore, do they? Yet there's millions of Flash animations and games never ported to another platform
I think the idea would be that when you click on a link to a 25 year old page, the browser brings up a little warning saying "This page is 25 years old and has some errors that may stop it displaying correctly. Press the Refresh button to re-render it using an older browser engine."
In the worst case, the browser would then have to download some additional code that contained all the support for invalid HTML/CSS code. In the best case, the 25 year old page would be served with a header containing a cryptographic proof that the site really had been around for 25 years, and it wasn't just some newly created attack site that was exploiting some weird behaviour in old renderers.
If your browser handles all of the popular sites, people will use it. Nobody cares about whether Flash loads anymore, do they? Yet there's millions of Flash animations and games never ported to another platform