They pay for content that can be rendered on their browsers. If that means it takes me more time because of IE's poor CSS support, or if I just don't make the site as good as I could have because I've dumbed-down the CSS for IE, then fine -- they're paying for it.
BTW, I started getting paid for doing web sites about the same time that BillG realized the WWW exists. I was still doing it five years ago when anyone willingly hosting on IIS was considered an idiot, and I'm still doing it now when Microsoft finally has decent server software, but a horrible client.
What happened to your "screw'em" strategy?