Detailed explanation of CSS3 flex box automatic filling writing

<style type="text/css">
    .flexcontainer{
        width:100%;
        height: 100%;
        position: absolute;
        left:0px;
        top:0px;
        display: flex;
        flex-direction: column;
    }
    .flex1 {
        width: 100%;
        height: 8rem;
        float: left;
        background: #f00;
    
    }
    .flex2 {
        width: 100%;
        height: 4rem;
        flex:1;
        float: left;
        background: #000;
    }
   .flex3 {
        width: 100%;
        height: 7rem;
        float: left;
        background: #ccc;
    }    
</style>
<div class="flexcontainer">
    <div class="flex1"></div>
    <div class="flex2"></div>
    <div class="flex3"></div>
</div>