See Flexbug #9: https://github.com/philipwalton/flexbugs#9-some-html-elements-cant-be-flex-containers
In http://codepen.io/anon/pen/MaYBoX , the children of the `display:flex` <fieldset> stack vertically, which is incorrect.
Compare this to the same code using a <div> instead of a <fieldset>,
where the children are correctly displayed horizontally: http://codepen.io/philipwalton/pen/EVaRaX
Created attachment 260618 [details]
Copy of the CodePen testcase
Filed a Radar:
*** This bug has been marked as a duplicate of bug 169082 ***