Most good webmasters try to test their websites for compatibility to the best of their abilities--we have several different browsers and have validated our code. However, being only one person at a time, there are only so many things we can do. What if we don't have a Mac? What if we don't have cell phones and PDAs with internet capabilities? What if we can only download the trial version of Opera so many times?
For those of us that can't cover all of our bases, this community exists as a place where we can come to get help from others in making sure our websites can be accessed by anyone.
This community is NOT, however, a place to discuss web standards, HTML, or webmastering in general; it is NOT a place to come and ask people to teach you HTML (unless you are having trouble with a compatibility problem), or a place to critique design (unless it gets in the way of accessibility). There are plenty of other communities for those sorts of things.
Feel free to join if you'd like to help others with web accessibility problems, or if you need help in this area yourself.
If you, like me, get especially irked by websites that don't care to be compliant and accessible, then you will probably enjoy browserfriendly, where you will be welcome to name and complain about them.