Browser - Backward Compatibility

In designing a site - which browsers should one design for? At what point does a website owner stop sinking funds developing and testing for out-of-date browsers? I don't have a clear cut answer for this. Is it solely market share? No. A browser may have a tiny market share but be web standards compliant. This means that it takes relatively little effort (read money) to make certain that nothing is broken and the site renders well.

It used to be that browsers ignored standards trying to create proprietary standards. Those days are gone forever but the browsers remain and some, such as IE6, are still running strong. This allows us to rephrase the question. At what point should site owners and developers stop validating non-web standard compliant browsers?

It is simply a matter of math. How much are you willing to spend validating a browser for 0.1%, 1%, 10% of your market? I would say that when the browser has declined to under 10% of your site usage that it would make sense to ignore minor visual inconsistencies and focus resources only on issues that prevent visitors from using your site.

EDIT 9/7/2010:

Sources for Web Statistics

www.w3schools.com/browsers/ www.upsdell.com/BrowserNews/