The biggest and most irritating challenge for a web developer today is to face the compatibility issues across multiple browsers and solving them. Expression Web SuperPreview is a tool coming with Expression Web 3 that enables web designers to check compatibility of their web sites in multiple browsers. It allows to debug the lags easily that are causing incompatibility.
Right now it’s only available for Internet Explorer: You can try by checking compatibility of your website in IE 7 and IE 8 with the version that is available right now.
Some screenshots from a newer build of Expression SuperPreview (not available for download right now) gives glimpse of more features. Expression SuperPreview allows you to monitor and test rendering of your website on different web browsers in a single unified interface. (Screenshots Posted by Long Zeng on his blog)
HTML debugging tools like a DOM inspector, CSS inspector, element highlighting, pixel rulers and guides are part of it. Simply once place to solve all your compatibility problems.
Overlay layout (That is in the current downloadable version) in the screenshot below tells the differences of website rendering on multiple platforms.
You can download the Beta here: Expression Web SuperPreview for Internet Explorer (250MB)