多个div占满整个屏幕（或页面）（待补充及理解）

    <div id=\"app\" class=\"outer\">
        <h3 class=\"head\">Head组件</h3>
        <div class=\"container\">
            <h3 class=\"left\">Left组件</h3>
            <h3 class=\"main\">Main组件</h3>
        </div>
    </div>

第一种方法：

        html,
        body {
            height: 100%;
            margin: 0;
            padding: 0;
        }

        .outer {
            height: 100%;
            padding: 100px 0 0;
            box-sizing: border-box;
        }

        h3 {
            margin: 0;
            padding: 0;
        }

        .head {
            background-color: antiquewhite;
            height: 100px;
            line-height: 100px;
            text-align: center;
            margin: -100px 0 0;
        }

        .container {
            display: flex;
            height: 100%;
        }

        .left {
            background-color: aqua;
            flex: 2;
            text-align: center;
        }

        .main {
            background-color: blueviolet;
            flex: 8;
            text-align: center;
        }

效果：

第二种方法：

  html,
        body {
            height: 100%;
            margin: 0;
            padding: 0;
        }

        .outer {
            height: 100%;
            padding: 100px 0 0;
            box-sizing: border-box;
            position: relative;
        }

        h3 {
            margin: 0;
            padding: 0;
        }

        .head {
            background-color: antiquewhite;
            height: 100px;
            line-height: 100px;
            text-align: center;
            position: absolute;
            top: 0;
            left: 0;
            width: 100%;
        }

        .container {
            display: flex;
            height: 100%;
        }

        .left {
            background-color: aqua;
            flex: 2;
            text-align: center;
        }

        .main {
            background-color: blueviolet;
            flex: 8;
            text-align: center;
        }

效果：如上图