css设置div自动填充剩余高度

html,
body { height: 100%; padding: 0; margin: 0; }
.outer { height: 100%; padding: 100px 0 0; box-sizing: border-box ; }
.A { height: 100px; margin: -100px 0 0; background: #BBE8F2; }
.B { height: 100%; background: #D9C666; }

<div class="outer">
    <div class="A"> 头部DIV </div>
    <div class="B">下部DIV </div>
</div>