Ideally IMO this would be accept headers. You're asking for the same semantic content but a different format. I'm not sure if there's a nice way of specifying html but in a minimal sense (we do quality with images, perhaps linked), however these could mostly be text/plain or text/markdown (and it'd be nice if that was then formatted properly by the browser).

This often makes a really nice API if you can do other formats too - the main page of cnn could respond to rss accept headers and give me a feed for example.