From the Expression Web team blog,

Expression Web SuperPreview for Internet Explorer is a free stand-alone visual debugging tool that makes it faster and easier to migrate your sites from Internet Explorer 6 to Internet Explorer 7 or 8. Expression Web SuperPreview for Internet Explorer shows your web pages rendered in Internet Explorer 6 and either Internet Explorer 7 or Internet Explorer 8, depending on which version you have installed on your machine…

 

Microsoft Expression Web SuperPreview for Windows Internet Explorer

 

Other important facts from that page:

  • Expression Web SuperPreview for Internet Explorer not only shows a high-fidelity rendering of how pages will look on different browsers, but it also identifies the element’s tag, size and position, applied styles, and location in the DOM (Document Object Model) tree so you can quickly fix the error.