Which browsers do you want your website to work on? All of them, of course.
Let me ask it a different way. After I make a website that follows all of the current standards and works perfectly on browsers that follow them, how much are you willing to pay to get it to work on browsers that don’t follow those standards and to check that it really does? How many phones and tablets (they use browsers, too) do you want checked?
There comes a point where you’ll decide that it isn’t worth checking how your site works on a browser that came out in 2001. (That may sound silly until you realize that I’m describing Internet Explorer 6. Please see the sidebar, How old is too old?)