is equivalent to:
> > Br / > if you want to transfer multiple attributes at the same time, you can use a comma, such as:
using the transtion attribute only specifies how to transition. To let the animation happen, you have to have a yuan. A change in the state of the vegetarian state. How to change the state of the element is, of course, to define a class in CSS (hover and other pseudo classes), which describes the state of the element at the end of the transition animation.
so, when we move the mouse to the div, the state of the div changes, and you can see the width from 100 to 400, the height from 100 to 400, the background color from black to red. The transition is 3 seconds.
in addition to using pseudo classes provided by systems such as hover, we can also define our own classes at will, and then add classes to the elements through JS when you want to transition: