语法:
background-image: linear-gradient(direction, color-stop1, color-stop2, ...);
/* 从上到下,蓝色渐变到红色 */
linear-gradient(blue, red);
/* 渐变轴为45度,从蓝色渐变到红色 */
linear-gradient(45deg, blue, red);
/* 从右下到左上、从蓝色渐变到红色 */
linear-gradient(to left top, blue, red);
/* 从下到上,从蓝色开始渐变、到高度40%位置是绿色渐变开始、最后以红色结束 */
linear-gradient(0deg, blue, green 40%, red);
0deg=to top;
90deg=to right;
180deg=to bottom;
-90deg=to left;