mysiteonline™

A couple years ago, Salted, the brains behind SiteVista, created an application to ease the pain of working with the compatibility differences in developing CSS-based websites between the two major browsers: Firefox and Internet Explorer. There were a few noted problems with that version, but it was still usable if you knew how to get around it all. But, now we don't have to deal with those problems anymore!
The newer version seems to have removed that problem and not introduced any more problems. Even better yet, it introduces many more features, such as code syntax highlighting (somewhat of an expected thing now-a-days), auto-complete, and more... It more or less implements the Web Developer Toolbar extension for Firefox in Internet Explorer (no easy feat for the element outlining, by any means) with syntax highlighting and auto-complete. The tool will eventually be developed in to a separate application so it will be the last version of CSSVista as it is now, though it should be available for free in some form or fashion.

To use it and develop your CSS, all you do is load up the page in the browser/application. You choose to "Edit CSS" and it will open up the CSS edit pane. You can then split the main window viewport to edit with both Firefox and IE in a split window application view. By enabling a live view-as-you-type approach to CSS editing in both browsers, it makes it much easier to develop cross-browser compatible CSS layouts with regard to the box model problems that each browser may show (almost entirely on IE's fault, of course).

I have noticed that the rendered display is not completely, 100% accurate, but I'm not sure if this is to due with CSSVista's implementation of the engines or with the external engines themselves.