AtoZ CSS Screencast: The ID Selector

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AtoZ CSS Screencast: The ID Selector

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This screencast is a part of our AtoZ CSS Series. You can find other entries to the series here.


ID is a CSS selector that allows the styling of a single unique element.

Its use in CSS is common and often a little controversial.

I personally avoid IDs for styling things in CSS, but I’m not going to force my personal preferences on you.

In this video, I will outline some of the reasons why I prefer the class selector over id, how CSS specificity works and some tips for writing more modular and reusable code.


Let’s start with the basics and look at how ID can be used to select an element in CSS.

Given this series of <div>s, if I wanted the third one to be a feature box, I could add an id attribute to the HTML as follows. This is a unique identifier, a bit like your passport number or social security number and there can be only one with this value on any given page.

<div>lorem ipsum</div> 
<div>lorem ipsum</div> 
<div id="feature-box" class="box">lorem ipsum</div> 
<div>lorem ipsum</div>

With the id attribute added in the markup, this can be selected in CSS using the hash symbol which outside of the UK is sometimes called the pound sign.

#feature-box {
  /* styles */

Now we know how to use the ID selector, let’s look at why I don’t use them.

The trouble with IDs

I’ll outline two key reasons why I avoid using the ID selector in CSS:

  • They are single use
  • They introduce specificity issues

Single Use

One of CSS’s strongest weapons is the class selector which allows you to style multiple things, multiple times with a single block of code.

I often use a class of “box” in the code demos of this tutorial series to apply generic text styling and color to the elements on the screen, so you can see what’s going on. I can re-use this class as many times as I need and can apply it to a whole host of different elements. The class selector doesn’t care what element you put it on.

If I had <div id="box"></div> I could only use this once. If i’ve gone to the trouble of tapping out a whole load of CSS, I at least want the option of using it again.

Thinking in reusable blocks can also help you break down lots of CSS into smaller chunks that have more likelihood of being reused.

To make your code more modular, consider breaking things down into separate classes for structure and look.

We could have a reusable button component with the following structural styles:

.button {
  display: inline-block;
  margin: 1em 0;
  padding: 0.5em 1.5em;
  font-size: 1em;
  text-align: center;

We could then create a series of modifier classes that allow us to create a range of different buttons for use throughout a project.

.button-large {
  font-size: 2em;
.button-rounded {
  border-radius: 1em;
.button-confirm {
  color: #333;
  background: lightgreen;
  border: 1px solid darkgreen;
.button-cancel {
  color: #fff;
  background: crimson;
  border: 1px solid darkred;

These modifier classes can then be added to the markup as needed to build the whole component up of many smaller pieces.

<a href="#" class="button button-large button-confirm">OK?</a>


Specificity determines what styles get applied and when.

When writing CSS, things that come further down the stylesheet tend to override things that have been declared above them.

.box p {color: white;}
.box p {color: black;}

The second set of styles override the first.

What color text will the box have in this snippet?

#box p {color: white;}
.box p {color: red;}

The text will be white because although the class selector comes after the ID selector, the ID is a more powerful selector.

How about now?

#box p {color: white;}
body h1 + p {color: green;}

The text is still white. This is due to how specificity is actually calculated.

Calculating Specificity

Take this horrifically complex selector – please never write anything like this.

header ul#main-nav li .sub-menu li.child a:hover { }

This will style the hover states of any links in a sub-menu of a site’s main navigation. Yuck.

To calculate the specificity of this selector – ie. what we would have to concoct to override these styles somewhere else in the stylesheet – we need to count all the style attributes, id selectors, classes and pseudo classes and elements to create a 4-figure number.

There are no style attributes here. So the first number is 0.

There is 1 ID for “main-nav” which must be on a ul for this selector to apply.

There are 2 classes, .sub-menu and .child and 1 pseudo class, :hover so the third number is 3.

And finally, there are 5 elements; header, ul, li, li again and a.

The specificity value for this selector is therefore

0, 1, 3, 5

In order to override these styles, we would need another selector with equal specificity to occur beneath this one in the stylesheet, or a selector with higher specificity to occur anywhere in the file.

To avoid this madness, the above selector could almost certainly be re-written as

.sub-menu a:hover { }

And had the same visual result with a specificity of 0,0,2,1 which would be much easier to deal with if needing to be overwritten at a later stage.

I like to keep my specificity low and favor classes wherever possible; it reduces dependency on certain markup structure and allows things to be easily overwritten with another single class selector if needs be.

This approach of avoiding id works for me, but your preference might differ. I’d encourage you to try this approach, but only as part of finding out what really works best for you and your team.

Watch out for our Quick Tip article coming soon!

Guy RoutledgeGuy Routledge
View Author

Front-end dev and teacher at The General Assembly London. A to Z CSS Screencaster, Founder of and Co-founder of The Food Rush.

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