var list = [];
document.querySelectorAll("body *")
   .forEach(function(elem){ 
    if(elem.getBoundingClientRect().width > document.body.getBoundingClientRect().width){ 
      list.push(elem.outerHTML.split('>')[0] + '>'); 
    } 
}); 
confirm( "these elements are wider than the viewport:\n\n " + list.join("\n") )