css左固定右自适应常用方法

<div class="demo">
<div class="sidebar">我是固定的</div>
<div class="content">我是自适应的</div>
</div>

.sidebar,.content{
height: 300px;
}
.sidebar{
background-color: red;
width: 300px;
}
.content{
background-color: green;
}

        .demo .sidebar{
            float: left;
        }
        .demo .content{
            margin-left: 310px;
        }

        .sidebar{
            position: absolute;
            top: 0;
            left: 0;
        }
        .content{
            margin-left: 310px;
        }

        .demo{
            display: flex;
        }
        .sidebar{
            width: 300px;
            margin-right:10px;
        }
        .content{
            flex: 1;
        }