background-image: linear-gradient(red, yellow, blue, green); 
background-image: linear-gradient(rgba(255, 0, 0, .2), yellow, blue, green);

/*
    控制颜色渐变的方向
    to right -- 从左向右
    to top -- 从下到上
    to left -- 从右到左
    to bottom --- 从上到下（默认值）
*/
background-image: linear-gradient(to right, red, yellow, blue, green);
background-image: linear-gradient(to top, red, yellow, blue, green);
background-image: linear-gradient(to left, red, yellow, blue, green);
background-image: linear-gradient(to bottom, red, yellow, blue, green);

/*0deg = to top -- 从下到上*/
background-image: linear-gradient(0deg, red, yellow, blue, green);
/*基于0度顺时针旋转45deg*/
background-image: linear-gradient(45deg, red, yellow, blue, green);
/*基于0度逆时针旋转45deg*/
background-image: linear-gradient(-45deg, red, yellow, blue, green);

/*设置过渡颜色的起始位置*/
/*从过渡起始位置50px开始让红色和黄色之间产生颜色渐变效果*/
background-image: linear-gradient(to right, red 50px, yellow, blue, green);
background-image: linear-gradient(to right, red 50px, yellow 50px, blue, green);
background-image: linear-gradient(to right, red 50px, yellow 50px, yellow 100px, blue, green);

background-image: linear-gradient(to right
, red 0
, red 50px
, yellow 50px
, yellow 100px
, red 100px
, red 150px
, yellow 150px
, yellow 200px);
/**与上面重复写渐变有相同的效果*/
background-image: repeating-linear-gradient(
        to right
        , red 0
        , red 50px
        , yellow 50px
        , yellow 100px
);