The difference between class and ID in Div+css _ Experience Exchange

A class is used to define one or more elements based on user-defined criteria. A more appropriate analogy is the script: A class can define the story lines of each character in the script, and you can use this class by Css,javascript. So you can use class= "Frodo" on one page, class= "Gandalf", class= "Aragorn" to distinguish different story lines. It is also very important that you can use any number of classes in a document.

As with IDs, it is often used to define a single mark on the page that appears only once. When you make a structured layout of page layouts (for example, usually a page is made up of a header, a header <masthead>, a content area and a footer), it is generally ideal to use IDs because an ID can only be used once in a document. These elements rarely appear more than once on the same page.

The sentence is: class can be used repeatedly and IDs can only be used once in a page. It is possible that using the same ID repeatedly in a large number of browsers will not cause problems, but it is definitely wrong to use on the standard and is likely to lead to some real-world problems with browsers. (Just tried it in Dreamweaver, yes, no mistake)

In practical applications, class may be useful for text typesetting, and ID is useful for macro layout and design placement of various elements.