The difference between CLASS and ID in DIV CSS web page production_html/css_WEB-ITnose

Reprinted from: http://blog.onlygrape.com/divcss-3/381

Recently, when I was doing an effect of making the background of a PNG background image transparent, I applied When it comes to ID, CLASS has been used when doing page effects before. Here we analyze the application of CLASS and ID when defining the page style sheet.

1. When writing in CSS files, the ID is prefixed with "#"; CLASS uses "."

How to use class and id in the page:

class：

< style type ="text/css" >
.header { background : red ; }


< div class ="header" > header

id:

< style type ="text/css" >
#header { background : red ; }


< div id = "header" > header

css that defines the class Use a dot: ".", such as .header
The css to define the id uses the pound sign "#", such as #header

2. The id can only be used once on a page; class can Quoted multiple times.

3. ID is a label used to distinguish different structures and contents

Just like names, if there are two people with the same name in a room, confusion will occur;
CLASS is a style that can be applied to any structure and content, just like a piece of clothing;

id can only be used to define a single element, after defining two. There will be no problems with the page, but when W3 detects it, it thinks your page does not meet the standards; class is a class, and the same class can define multiple elements.

As far as page effects are concerned, the visual effects of the two things are almost the same.

4. Conceptually, they are different:
id first finds the structure/content, and then defines a style for it; class defines a style first, and then Set for multiple structures/contents.

A Class is used to define one or more elements based on user-defined criteria. A more appropriate analogy is a script: a Class can define the story line of each character in the script. You can use this class through CSS, javascript, etc. So you can use class="Frodo", class="Gandalf", class="Aragorn" on a page to distinguish different story lines. Another very important point is that you can use Class any number of times in a document.

As for ID, it is usually used to define a tag that only appears once on the page. When making a structured layout for page layout (for example, a page usually consists of a header, a masthead>, a content area and a footer, etc.), it is generally ideal to use ID, because an ID is Can only be used once in a document. These elements rarely appear more than once on the same page.

It can be summed up in one sentence: Class can be used repeatedly but ID can only be used once in a page. It is possible that using the same ID repeatedly will not cause problems in most browsers, but this is definitely an incorrect use according to the standard, and may cause real-life problems in some browsers.

In practical applications, Class may be more useful for text layout, etc., while ID is more useful for macro layout and design placement of various elements.