There is often confusion when it is appropriate to use CSS IDs and when CSS Classes should be used instead. This lesson is geared to display an answer as well as provide more information about CSS IDs

What is an ID Selector?
The W3C defines class ID as "a unique identifier to an element". But what does that actually mean? Hopefully you have already read our CSS Classes lesson before continuing, if not, we recommend doing so.

CSS IDs are similar to classes in that they define a special case for an element. In other words, they assign an identifier. Below is an example of a CSS ID.

CSS Code:
p#exampleID1 { background-color: white; }
p#exampleID2 { text-transform: uppercase; }

HTML Code:
<p id="ExampleID1">This paragraph has an ID name of "exampleID1" and has a white CSS defined background</p>


<p id="ExampleID2">This paragraph has an ID name of "exampleID2" and has had its text transformed to uppercase letters. </p>
Display:
This paragraph has an ID name of "exampleID1" and has a white CSS defined background.

This paragraph has an ID name of "exampleID2" and has had its text transformed to uppercase letters.

Notice that an ID's CSS is an HTML element, followeb by a "#", and finally ID's name - "element#idname". Also, be sure to absorb the fact that when an id is used in HTML we must use "id=name" instead of "class=name" to reference it!

Why Did They Choose Those Names??
ID = A person's Identification (ID) is unique to one person.
Class = There are many people in a class.
ID for Layout and Uniqueness
Standards specify that any given id name can only be referenced once within a page or document. From our experience, IDs are most commonly used correctly in CSS layouts. This makes sense because there are usually only one menu per page, one banner, and usually only one content pane.

In Tizag.com CSS Layout Examples we have used used IDs for the unique items mentioned above. View the CSS Code for our first layout example. Below are the unique IDs in our code.

Menu - div#menuPane
Content - div#content
Answer: Classes vs IDs
Use IDs when there is only one occurence per page. Use classes when there are one or more occurences per page.










http://www.tizag.com/cssT/cssid.php