[porkloin]

I sincerely hope BS4 is the last nail in IE9's coffin.

[jordanlev]

Out of curiosity, is there anything significant other than flexbox that's not in IE9 but is in IE10/11?

[acdha]

According to http://caniuse.com/#compare=ie+9,ie+10 there's a bunch of the HTML5 form stuff, reliable use of CSS calc(), web workers, WebVTT (subtitles for video), classList, etc.

[mishac]

Whole bunch of stuff: http://caniuse.com/#compare=ie+9,ie+11

CSS Transitions is probably the biggest one, but there's a lot, and a ton of javascript improvements too.

[axlee]

I'm pretty sure IE9 does not support CSS transitions or CSS animations in general. There is also a lack of support of most CSS gradients. Websockets and web workers aren't supported either. A lot of HTML5 form elements are also missing.

[intrasight]

I sincerely hope BS4 is the last nail in IE10's coffin