Accessibility is the law in many countries now. If your site is not accessibile, you run the chance of maybe being charged. This is even more evident now with many large companies being sued or settling out of court, for not having accessible websites. One would hope that most people by now are aware of accessibility & usability when it comes to websites, but there still are some shockers out there that come no where near it!
In most cases it takes very little effort to make a site even "A" rated (the lowest one - Priority 1) in accessibility. There are many finer points, and many arguable ones .... but an attempt should at least be made.
For usability, at least try for things being in a logical order, even if you cant get it pixel perfect. Having text disappear under images is just tacky. Your site is of no use at all to people who cant read it.
No ones asking for perfect .... just an attempt.
You might find it better to check these things early in construction, because you can save yourself a lot of changing if you catch it early.
