Focus on speed and modularity next to the clear code sounds interesting, but supporting IE 9+ only is kind of a bummer here, because it means "no" for XP users (I use XP64 on my desktop for instance) and there is still substantial number of them.
(And I do curse Microsoft for making IE 9 Vista+ only browser, which is quite likely explainable by use of some shiny new undocumented API, that wasn't available in XP and before. Microsoft and their browser tightly coupled with OS...)
It does support it if you include es5-shim, though IE8 is effectively dead. It's at < 5% share globally, around the same point we all dropped IE6 in the past, and effectively at 0% for any web service / startup out there. It's commonly surpassed by visits from Safari/iOS/Android.
Firefox, Chrome and Opera are available for Windows XP :)
I wonder if libraries from Wine or Reactos could be used to run IE9 on XP.
I seriously doubt the reasons Microsoft gave for not supporting it are valid. Even if the sandbox mode could not be used there is no reason why the browser and rendering engine could not run on XP if the proper system DLLs were present, and these could have easily been included with the install as Microsoft does in many other cases.
(And I do curse Microsoft for making IE 9 Vista+ only browser, which is quite likely explainable by use of some shiny new undocumented API, that wasn't available in XP and before. Microsoft and their browser tightly coupled with OS...)