They had an actual shot on getting up to par with proper HTML5, with funky ms extensions being optional where absolutely neccessary. Instead, they rely on an astounding number of non-cross-browser methods and features, as they have done since '95
https://github.com/winjs/winjs/issues Most issues listed currently are about stuff that just doesn't work on any other browser/platform than IE
Footnote: I'm a web developer myself and I use Windows 8.1. How tempted have I been at fiddling with the winjs flavour myself? not at all.
Footnote2: I do have to give them credit for the roadmap though. It seems like they're on the right track https://github.com/winjs/winjs/wiki/Roadmap
Footnote3: I've been fooled into thinking that Microsoft was on the right track again and again.