in Technology

CSS classes and IDs, Which one to use when?

I was trying out a way to float 4 DIVs inside a parent DIV on a quick site design, and this topic came up to my mind. I even explained this to a group of attendees at the India MAX 2004, Bangalore. Many times in the past, I have tried to get this archived but somehow slipped my mind, so, here we go.

Classes and IDs, which one to use, when and why?

By now, it is rather a common thing, most (x)html/css layout designers know very well where to separate ID selectors from classes in CSS! This article should most aptly apply to newcomers.

The W3C defines that attributes of the type ID cannot have the same value more than once in a document, which means that it is unique for that particular document. Nonthless, as of today, most browsers will not complain even if you violate the same. It may be noted that a CSS ID Selector usually contains a “#” followed by the ID value.

#myid {
	position: relative;
	margin: 0 auto;
	padding: 0;
	text-align: left;
	background: url(../images/pattern.gif) repeat-y;
}

or

div#myid {
	position: relative;
	margin: 0 auto;
	padding: 0;
	text-align: left;
	background: url(../images/pattern.gif) repeat-y;
}

Classes on the other hand can be used many times in a document. So, they are best used to define a common style which can be used on multiple elements on your document. They start with a . (dot) followed by the value.

.myClass {
	font: normal normal normal 86%/1.6em "Lucida Grande", "Lucida Sans Unicode";
	border-top: 1px solid #ebebeb;
	text-transform: lowercase;
	color: #666;
}

An element can have an ID selector and mutiple classes applied to it. Remember one important thing here, ID selectors have higher precedence over class selectors.

So, IDs are for specific unique elements in your particular document and classes to be applicable to multiple elements in a document.