Q:

Why are not all flexbox elements behaving like flexbox divs?

Why is flexbox not working properly with fieldset or other non-div tags?
I expect them to line up next to each other like in the div example, as flex-direction: row; is default in flexbox. However fieldset is force applying a width to them, and I do not understand why.
HTML
<fieldset>
    <div>fieldset flexbox</div>
    <div>1</div>
    <div>2</div>
</fieldset>

<div id="parentdiv">
    <div>div flexbox<div>
    <div>3</div>
    <div>4</div>
</div>

CSS: All elements are set to display: flex;
http://jsfiddle.net/c5BB5/1/

A:

As far as I can tell, this is down to browser bugs to do with the fieldset element.
It's a known issue with fieldset elements in Chrome. Firefox has a similar (very old) issue in that legend and fieldset are replaced elements.

I guess it's safer to use a <div role="group"> instead of a real fieldset for now. In your CSS you could use div[role='group'] as your selector. See http://www.deque.com/aria-group-viable-alternative-fieldset-legend for more information.

