<style> 
#div1
        {
            position: relative;
            height: 300px;
            width: 90%;
            background-color: skyblue;
        }
        #div2
        {
            position: absolute;
            bottom: 0;
            height: 30px;
            width: 100%;
            background-color: lightgreen;
        }
    </style>

<div id="div1">
	<div id="div2"> 无论蓝色div高度如何变化，绿色div都会贴在下面
	</div>
</div>