前端二倍圖和三倍圖的自適應調用

/* 如果設備像素大於等於2，則用2倍圖 */
@media screen and (-webkit-min-device-pixel-ratio: 2), screen and (min--moz-device-pixel-ratio: 2) {
  .img {
    background-image: url(../images/[e-mail]);
    background-size: 30px 30px;
    width: 30px;
    height: 30px;
    background-repeat: no-repeat;
  }
}
/* 如果設備像素大於等於3，則用3倍圖 */
@media screen and (-webkit-min-device-pixel-ratio: 3), screen and (min--moz-device-pixel-ratio: 3) {
  .img {
    background-image: url(../images/[e-mail]);
    background-size: 30px 30px;
    width: 30px;
    height: 30px;
    background-repeat: no-repeat;
  }
}