Some of the concepts in CSS

1. Pseudo-class and pseudo-elements

1, single colon (:) for CSS3 pseudo-class, double colon (::) for CSS3 pseudo-elements. 2. For pseudo elements already in CSS2, such as: before, single colon and double colon: before function is the same.

3, so, if your site only need to be compatible with WebKit, Firefox, Opera and other browsers, it is recommended to use a double colon for pseudo-elements, if you have to be compatible with IE browser, or with CSS2 single colon is more secure.

4, , <input>, <iframe>, these tags are not supported similar to img::before such use (to want the tag to support pseudo-elements, need this element is to be able to insert content, Which means that this element is a container. Elements such as Input,img,iframe cannot contain other elements, so the content cannot be inserted by pseudo-elements)

Some of the concepts in CSS