Pseudo element clear float (parent element height collapse)

Clear float:

The following image is height collapse: the parent element has a height of 0 because the child element is set to float

1. You can use pseudo-elements to clear the float:

This method is invalid in ie6, 7, you need to set zoom:1 to #content.

#content{zoom:1}

2. Add a child element to the parent element clear:both; (inner wall method)