笔记 ： CSS3实现背景渐变过渡

　　使用CSS3的人都知道背景background-image是可以线性渐变（linear-gradient）和径向渐变（radial-gradient），但是想要做到过渡动画，单纯的background-image是不够的

　　（1）/* 借助background-position */

/****** CSS *****/

.position {
    width: 400px;
    height: 200px;
    background: linear-gradient(to right, red, yellow);
    background-size: 200%;
    transition: background-position 1s; 
        }
.position :hover {
    /*默认是0,0*/
        background-position: 100% 0;
} 

/***** HTML *****/
<div class="position"></div>

　　　（2）/* 借助background-color */

/**** CSS ****/
.color{
    width: 400px;
    height: 200px;
    background: olive linear-gradient(to right, rgba(0,255,255,0), rgba(0,255,0,.5));
    background-size: 200% 0;
    transition: background-color 1s; 
}
.color:hover {
    background-color: pink;
}

/**** HTML ****/
<div class="color"></div>

　　　　（3）/* 借助伪元素 */

/**** CSS ****/
.opacity{
    width: 400px;
    height: 200px;
    background: linear-gradient(to right, red, yellow);
    position: relative;
    z-index: 0;
}
.opacity::before{
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    background: linear-gradient(to right, yellow, pink);
    opacity: 0;
    transition: opacity 1s;
    z-index: -1;
}
.opacity:hover::before {
    opacity: 1;
}

/**** HTML ****/
<div class="opacity"></div>