Well, I made some code that would emulate flexbox in IE for a specific HTML structure (it would parse classnames in the HTML and not the CSS, the code is still around on https://github.com/wiredearp/spiritual-mix/blob/master/dist/...) but it turned out to run about a hundred times faster than the associated CSS flex that other browsers would use; even in those browsers. The layout must aspire to a certain complexity before it happens, that's why you don't notice it at first (and why I posted the comment). I just checked back with the issue called "Catastrophic flexbox perf" over on https://code.google.com/p/chromium/issues/detail?id=331352 and it appears that it's now a lot better in Chrome, but you can try to Ctrl+F for "hosted demo" on that page and open the demo in Firefox (you should save your work first!). When the page eventually does appear, you'll see that it's really nothing special, and certainly not worth the wait.