CSS was designed for running on different UAs and there is special mechanism built in it. If you take a look into source of that demo page, you will see:

   /* browser */
   @supports (display:flex)  
   {
     section { display: flex; }
     section > * { flex: 1; margin:1em; } 
     ...  
   }

   /* sciter */
   @supports (flow:horizontal)
   {
     section { flow:horizontal; }
     section > * { width:1*; margin:1em; }
     ...
   }

You should not assume that all browsers support same set of features. And needless to say, that Sciter is not a universal browser. And so is Cobalt in that respect - it is a browser for particular site / application.

> there is special mechanism built in it

Yes, a return to the dark days of UAs having their own proprietary extensions / prefixes to do the same thing. Why bother when there's an actual industry standard?

> You should not assume that all browsers support same set of features.

You can certainly target only user agents which support CSS Flexbox. I would even say it's the advisable default unless you have some very special needs.