如何使用CSS和HTML使主要内容div填充屏幕高度

.mainbody{     position: absolute;//here we are setting the position of an element as absolute   top: 30px; /* here we are defining Header Height to 30 px */   bottom: 10px; /*here we are defining  Footer Height to 10 px */   width: 100%;// here we are setting the width to 100%}