大螃嗨

redSpan: {
        position: 'relative',
        textDecoration: 'none',
        fontSize: 20,
        color: '#333',
        "&:hover": {
            "&:before": {
                width: '100%',
                left: 0,
                right: 0,
            }
        },
        "&:before": {
            content: '""', //这一行是重点
            position: 'absolute',
            left: '50%',
            bottom: -2,
            width: 0,
            height: 2,
            background: '#4285f4',
            transition: 'all .3s',
        },
 
    },