Very little of anything that uses JavaScript. IE6 doesn't support ES5, so huge amounts of very trivial code today would simply not run. IE6 also supports very little (no?) CSS2, so you'd be lucky if anything rendered sensibly even if the JavaScript was fine.

I imagine lack of support of TLS (by default configuration) would mean very little of the modern, encrypted web would work with it. (Out-of-the-box, the latest thing it supports is SSLv3.)

As a comparison point, I've an old 1st gen iPad (iOS5) that I use from time to time to browse the web with JS disabled. There are occasional layout issues, blank screens (JS-based websites), and out of memory-related crashes (too many large images), but most sites actually work fine.

iOS5 is not as ancient as IE6, but I'd nonetheless expect something similar holds for IE6 with a few more broken layouts and less crashes. One of the sibling comments might have a point with TLS though. Speed might also be a concern.

Quite a lot, actually. Most of the modern HTML5 is geared towards web and mobile apps, really. You can build most sites with old HTML4. Unless peeps are using a big-name CMS, most sites are still running on old HTML standards.

Some portions would work just fine :)

http://mincss.com

(disclosure: I'm the creator)

Few years ago I booted a Sun sparc with java hotspot browser. I could use google