From W3C Wiki
Why doesn't my web site display correctly in browsers when I have obeyed web standards?
Both web browsers and web standards are constantly evolving and updating, but unfortunately some web browsers are rather worse at keeping up with the evolution of the standards than others. The result is that pretty much no web browser will ever adhere to ALL the standards. To compound this problem, some browser authors think they have better ideas than some of the standards and choose to do things differently, or simply make a bad job of taking on some of the standards, meaning they only half-work, or work in completely the wrong way.
At the end of the day, if you've made a 100% standards-compliant web site, that should be enough and you should feel very satisfied; unfortunately it isn't enough because just about everyone looking at your site won't be seeing what you wanted them to see. You may, therefore, find you have to make a few concessions to accommodate some of the more popular browsers with poor standards support.
Don't just leave it at that, though - lobby the browser makers to improve their support for standards, and encourage your friends to try some of the more standards-compliant alternative browsers too, so that the industry can see that there is a demand for standards support.