Either support the W3C standard as a minimun, which just about all browsers handle well, or keep up supporting only a single version of only a single browser and tell the rest of your visitors what they can go do with themselves. It's really that simple. Accessibility doesn't mean trying to do something as useless as showing JPEGs to Helen Keller, but it does mean providing sensible alternatives or letting visitors with incapable browsers know what exactly they're missing and why.

As far as accessibility goes, the Kareha powering this board does a very good job: the only thing I found lacking is that deleting posts on a stock Kareha install must use JS. Links can do it, but Lynx can't.