全屏布局就是指html页面铺满整个浏览器窗口，并且没有滚动条，而且还可以跟随浏览器的大小变化而变化
<header></header> <div class="content"> <div class="left"></div> <div class="right"></div> </div> <footer></footer>
html,body{
margin: 0;
overflow:hidden;
}
header{
height: 100px;
position: fixed;
top:0;
left: 0;
right: 0;
background-color: lightgray;
}
.content{
position: fixed;
left: 0;
right: 0;
top: 100px;
bottom: 100px;
overflow: auto;
}
.content .left{
width: 300px;
height: 100%;
position: fixed;
left:0;
top: 100px;
bottom: 100px;
}
.content .right{
height:1000px;
margin-left:300px;
}
footer{
height: 100px;
position: fixed;
bottom:0;
left:0;
right:0;
background-color: lightgray;
}
