Agree that emulating browser type rendering isn't necessarily good in the long term, but why not build in support for a variety of different type rendering engines? Adobe is halfway there with Smooth, Strong, Crisp, etc... they could simply add options for WebKit, Firefox, IE...

Also, a web-focused app may not be necessary, but I think a digital-focused app is (well, I guess that would be Fireworks). Photoshop and Illustrator really are terrible tools for designing digital applications, for the simple fact that working with pixel-level detail is a pain in the ass (e.g. try making a 92px by 147px rectangle with no sub-pixel edges in one of those applications). Fireworks actually comes pretty darn close to what Jason is asking for, but Adobe seems to neglect its core functionality in favor of expanding on useless export options and plugins.