【CSS Transition】
CSS transitions provide a way to control animation speed when changing CSS properties. Instead of having property changes take effect immediately, you can cause the changes in a property to take place over a period of time.
div
{
width:100px;
transition: width 2s;
}
transition: property duration timing-function delay;
参考：
1、https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Transitions/Using_CSS_transitions