先演示再说结论：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<style>
#box1{
width: 100px;
height: 100px;
background-color: coral;
padding:10px;
border:10px red solid;
}
</style>
<script>
window.onload = function(){
var box1 = document.getElementById("box1");
alert("hello");
console.log(box1.style.width);
console.log(box1.clientWidth);
console.log(box1.offsetWidth);
console.log(box1.width);
}
</script>
</head>
<body>
<div id="box1" style="width: 100px;height: 100px;"></div>
</body>
</html>
输出：
说明如下内容，三个属性表示的位置：
即：
style.width = content的width
clientWidth = width + padding-left + padding-right
offsetWidth = width + padding-left + padding-right +border-left +border-right
