CSS Best-Practice: Selectors or Classes / IDs? [closed]

Question

What is the truly a best-practice in CSS: Using IDs and Classes or Selectors?

I have a stylesheet that is required by PHP once a form is submitted to style the results / successful-submission page and I was wondering what is a best-practise in CSS to use IDs and Classes or to use Selectors ?

Answer 1

Basically classes should be used for common page objects that may or could be repeated. ID's should only be used when there is a unique element on a page that you will not have anywhere else on the page.

One way to explain this would be:

• ID = A person's Identification (ID) is unique to one person.
• Class = There are many people in a class.

There are no browser defaults for CLASSES and ID's

Adding a class name or ID to an element does nothing to that element by default.

This is something that snagged me as a beginner. You are working on one site and figure out that applying a particular class name fixes a problem you are having. Then you jump over to another site with the same problem and try to fix it with that same class name thinking the class name itself has some magical property to it only to find out it didn't work.

Classes and ID's don't have any styling information to them all by themselves. They require CSS to target them and apply styling.

ID's have special browser functionality

Classes have no special abilities in the browser, but ID's do have one very important trick up their sleeve. This is the "hash value" in the URL. If you have a URL like http://yourdomain.com#comments, the browser will attempt to locate the element with an ID of "comments" and will automatically scroll the page to show that element. It is important to note here that the browser will scroll whatever element it needs to in order to show that element, so if you did something special like a scrollable DIV area within your regular body, that div will be scrolled too.

This is an important reason right here why having ID's be absolutely unique is important. So your browser knows where to scroll!

Elements can have BOTH

There is nothing stopping you from having both an ID and a Class on a single element. In fact, it is often a very good idea. Take for example the default markup for a WordPress comment list item:

<li id="comment-27299" class="item">

It has a class applied to it that you may want for styling all comments on the page, but it also has a unique ID value (dynamically generated by WordPress, nicely enough). This ID value is useful for direct linking. Now I can link directly do a particular comment on a particular page easily.

CAUTION: Be aware, Javascript does care

JavaScript people are already probably more in tune with the differences between classes and ID's. JavaScript depends on there being only one page element with any particular, or else the commonly used getElementById function wouldn't be dependable. For those familiar with jQuery, you know how easy it is to add and remove classes to page elements. It is a native and built in function of jQuery. Notice how no such function exists for ID's. It is not the responsibility of JavaScript to manipulate these values, it would cause more problems than it would be worth.

If you don't need them, don't use them

As you can see, classes and ID's are very important and we rely on them every day to do the styling and page manipulation that we need to do. However, you should use them judiciously and semantically.

This means avoiding things like this:

<a href="http://css-tricks.com" class="link">CSS-Tricks.com</a>

We already know this element is a link, it's an anchor element! No particular need here to apply a class, as we can already apply styling via its tag.

Also avoid this:

<div id="right-col">

ID is appropriately used here as the page will likely only have a single right column, but the name is inappropriate. Try and describe the context of the element, not where it is or what it looks like. An ID here of "sidebar" would be more appropriate.