WHAT TO DO WITH THIS?
Drag-resize your browser window to see how your HTML looks in different resolution. The ruler simply shows you what's the size of the window.
WHAT'S WITH RED AND GREEN RECTANGLES?
HOW TO USE
background:url(http://www.prowebdesign.ro/screen-ruler/screeen-ruler.png) top left no-repeat;}
WHY TO USE, if there are so many cool JS resizers and IFRAME tools out there?
As you probably know, there's no way to accurately test site for mobile environment on desktops or laptops. The only way to test accurately is to get phone/tablet you'r testing for. But since you can go broke buying so many gadgets, desktop testing is the next best thing:)