水平居中对齐
1、 容器: display: flex; justify-content: center;align-items: center
2、父：position: relative; 子：postion: absolute; top:50%; left:50%; transform: translate(-50%,-50%);
左边固定右边自适应
1、容器： display: flex;left: flex: 0 0 300px;right: flex: 1 1 auto
flex是flex-grow flex-shrink flex-basis简写
