css实现左侧固定宽度，右侧宽度自适应

#centerDIV {
height: 550px;
width: 100%;
}
#mainDIV {
height: 100%;
border: 1px solid #F00;
margin-left: 102px;
}
#leftDIV {
float: left;
width: 100px;
height: 100%;
border: 1px solid #F00;
}

<div id="centerDIV">
            <div id="leftDIV"></div>
            <div id="mainDIV"></div>
</div>

左侧leftDIV的float:left;width固定,右侧的mainDIV用margin-left