The class and id of the element and the classid of the element.

The class and id of the element and the classid of the element.
One element can have only one id. In the full text, this id can only appear on one element. One element cannot be written multiple times, for example, if you write <div class = "a" class = "B"> </div>, the 2nd classes do not take effect (this problem occurs because when I write tags, there are too many attributes. If you forget to define a class, you can write another one, so the subsequent class does not take effect.) The result is as follows: