Browser support is the key issue to understanding how to use Cascading Style Sheets effectively. Older browsers ignore CSS, compliant browsers support CSS, and other browsers provide either broken or quirky support for the standards. Knowing how to deal with the different types of browsers makes your style sheets more effective across a variety of platforms and browser versions.
The layout engine is the most important part of the browser, from a CSS perspective, as it determines how a web page is displayed—and thus how web designers deal with that browser. The Acid2 test was developed as one benchmark to measure a browser's conformance with CSS.
Internet Explorer (for Windows) is an example of a quirky browser—one that requires special attention from the web designer. Firefox, Opera, Safari, and related browsers are generally compliant with the web standards. Web designers need to be aware of these browsers and others when using CSS.