I've touched on this in the past, but to reiterate it is very frustrating to visit the Tech Preview site in IE 9 or IE 10 in cases where IE 11 is not made available + have the site detect the user agent + not even allow the page to attempt to render in the browser or to view the video preview of a synth.
True, these older browsers do not have WebGL, but they certainly can view profiles + have HTML5 video support. (Even if they didn't, you already have a Flash video player for video previews which you use on Facebook which could be used on desktop).
To be disallowed to even render a profile page or the Home page or the About or Help pages on earlier Microsoft devices such as early Windows Phone handsets, but to have the same page load on an older Android or iOS device is extremely irritating.
When auditing Tech Preview pages with http://modern.ie they suggest using a third party framework such as Modernizr or just rolling your own feature detection code.