.class1 {
  width: 100px;
  transition: width 2s;
  -moz-transition: width 2s; /* Firefox 4 */
  -webkit-transition: width 2s; /* Safari 和 Chrome */
  -o-transition: width 2s; /* Opera */
}
.class2 {
  width: 20px;
}

.class1 {
  width: 100px;
  height: 100px;
  transition-property: width, height;
  transition-duration: 1s, 1s;
}
.class2 {
  width: 20px;
  height: 20px;
}