css：让一个内部盒子沾满父盒子的剩余空间

<div class="app">
	<div class="header"></div>
	<div class="content"></div>
	</div>
</div>

.app {
	width: 100%;
	height: 100%;
	display: flex;
	flex-direction: column;
}

.header {
	height: 40px;
	background-color: red ;
	flex-shrink: 0;
}

.content {
	flex: 1;
	background-color: #000;
}