For years, web developers have complained that Internet Explorer didn't follow standards.
Starting in IE8, Microsoft made a huge effort to turn IE into a standards-compliant browser. With the notable exception of WebGL, a graphics standard that runs code directly on your graphics card which Microsoft has rejected for security worries, IE10 does a great job on standards that are actually part of HTML5 and mature enough to not need developers to keep rewriting their sites. It doesn't have all the experimental prefixes that testing sites that focus on WebKit give points for (but then neither do many of the websites you actually use).
The problem is that even though IE10 (and the version of IE10 on Windows Phone 8) can render pretty much everything WebKit based browsers can, it often doesn't get the chance. Web developers who've mistaken WebKit for the open web don't bother coding the advanced features of their site for anything but WebKit and they send IE users to 'downlevel' versions of the site intended for IE8 and 9.
In some cases, they do their sums so badly that they mistake IE 10 for IE 6. In others, they declare that the site won't work in IE at all. It's always instructive in those cases to use the developer tools in IE to fool the site into thinking you're using Chrome or Firefox to see how much site actually works.
The version of IE 11 in the leaked build of Windows Blue doesn't do quite that, which is a good thing: browsers trying to interpret prefixes marked for other browsers is not the way to get well-built web pages that take advantage of standards.
What is seems to do (remember, this is an unofficial leaked build), is to use a brand new user agent string: IE instead of MSIE. Developers can still target IE specifically, but IE 11 won't be hampered by being sent to versions of pages designed for old builds of IE with bugs long since fixed.
It's a smart idea. So smart, that it's what BlackBerry is doing in the browser in BlackBerry 10 - and for the exact same reason.
"We changed our user agent so it doesn't say BlackBerry but just BB," app platform product manager Tim Neil told me at the BlackBerry Jam event earlier this year. That avoids BlackBerry 10 users getting routed to sites designed for older BlackBerry browsers, or to a generic mobile site with fewer features when the BlackBerry 10 browser - which has the highest score on the HTML5 test site because it uses a very recent version of the WebKit engine - can actually load the WebKit-optimised sites usually reserved for iOS and Android.
"If you don't recognise us, at least give us the desktop site," says Neil. "Maybe the site you're visiting has a great mobile jQuery view; we can use the new user agent to get that to you."
It would be nice if more web developers didn't limit their worldview to WebKit (which isn't the de facto standard many assume it to be; see quite how different the different browsers using WebKit actually are at the useful Quirks Mode site). But changing the browser user agent is quite a creative way of shaking off the browser hacks and workarounds of the past.