A must read for all Web/SharePoint developers.
Internet Explorer 7 contains a number of improvements to cascading style sheet (CSS) parsing and rendering over IE6. These improvements are aimed at improving the consistency of how Internet Explorer interprets cascading style sheets as recommended by the W3C in order that developers have a reliable set of functionality on which to rely.
In some cases a few of these changes may have the effect of making existing content render in ways that are not compatible with IE6. This is often seen with elements moving to a different area of the page or overlapping content when viewed in IE7. These issues are most common on content that is using particular CSS constructs (often know as “hacks” or filters) to work around bugs that existed under the strict mode in IE6. In this article we’ll discuss why pages might be broken due to updated CSS support and the best ways to address the issues that result from it.
Read the entire Internet Explorer 7 Cascading Stylesheet Compatibility article