how to get html body size with jss code example

Example 1: how to give a background color strip in html and css

#colorstrip{
    width: 100%; height: 2px;
    border-style: solid;
    border-color: white;
    background-color: white;
}

Example 2: javascript get image dimensions

var img = document.getElementById("myImageID"); 
var imgWidth = img.clientWidth;
var imgHeight = img.clientHeight;