The world is full of unreadable websites: sites that have poor contrast such as dark gray text on a light gray background; sites that have tiny fonts; sites that obscure their content in a sea of advertising and widgets, sites that block their content with popup advertising...
These unreadable websites are especially painful for older people to deal with, since often their eyesight isn't as good as it used to be, and not being as tech savvy they are more easily confused by clutter and distractions. But the older demographic isn't a bad one for startups to target though, since one thing older people do have is money.
This startup idea is simple: create a bookmarklet (see, for example, http://www.google.com/bookmarks or http://ycombinator.com/bookmarklet.html) labeled "Readable". When the user gets to an unreadable page, they click on your "Readable" button in their bookmarks toolbar, and your site renders for them a readable version of the page that they're on.
You can include a small, tasteful header at the top of the page which includes a "Nope, not good!" button which gives you immediate feedback on which pages you are messing up when you try to render them in a readable fashion.
The core algorithm could be pretty simple: scan the page for the larger blocks of natural language text and display that. You don't have to be perfect, since if you miss anything of importance on the page the user can always click the "back" button to return to the original.
