Terry if you use Google, IE or Edge, press F12 to get to the Developer Tools.
In Google at least, there is a Toggle device toolbar (Ctrl+Shift+M) where you can pick the Responsive device form the drop down.
A quicker way is just to resize the browser viewport size.
Xara's approach includes all the Variant codes and images in one large package.
That is your killer.
In the Developers Tool, there is a Performance tab, with which you can have fun.
I also check the Sources tab.
Have fun,
Acorn
Bookmarks