An Intro to Object-Oriented CSS

After working with an object-oriented CSS framework over on, it's almost impossible for me to write CSS any other way. Thinking about CSS in an object-oriented fashion totally changed my way of thinkng about styling. Instead of constantly worrying about conflicting styles and patching them with more specific selectors, I started seeing ways to encapsulate my styles in reusable objects, and in doing so, I began to see how CSS could actually be fun and a little therapeutic.

Object-Oriented CSS Basics

The real leader of the OOCSS movement is Nicole Sullivan, who says that the two main principles of OOCSS are to separate structure and skin and to separate container and content. To me, this seems like a requirement of any well-written object-oriented code. In essence, you want loosely coupled code that is self-documenting. By separating structure from skin and container from content, we're able to reuse many components of CSS. We can cut down on repetitive styles, saving ourselves potentially hundreds or thousands of lines of code.

There are a few strategies to accomplish this decoupling, including preferring classes over ids as selectors, using BEM syntax, writing SASS or SCSS and compiling it to CSS later, and breaking down a design into small components.

BEM Syntax

BEM syntax complements OOCSS by breaking styles down into blocks, elements, and modifiers.

Blocks are standalone entities on a page -- modules, buttons, banners, etc. They are the parent-level objects or containers that would form the basis of your design.

Elements, meanwhile, are subcomponents of your blocks. They have no standalone meaning and are tied to their parents. They can also encapsulate specialized styling. Elements help you break down a block into additional parts that can be styled differently from the rest of the block.

Modifiers are flags on blocks or elements. They signify modifications to the base block or element. Think color changes, or shapes.

A block would be something like a .block. The element would be denoted by underscores .block__element. A modifier would be denoted by two dashes, .block--blue or block__element--round.

You can read more about BEM Syntax here.


Sass also helps you write OOCSS. Personally, I prefer the SCSS variant because I find the use of curly braces clearer. Whichever syntax you choose, Sass allows you to use variables, mixins, inheritance, nesting, and partials. You can make a stylesheet for each of your components and import all of these files into one manifest. Doing so reduces cognitive overhead and makes your CSS more modular and maintainable. I found it very useful to store colors in variables in order to keep the colors used in the design consistent. Variables are also great for assigning meaning to seemingly arbitrary numbers. You could compose your framework around a specific pixel number and then make your object sizes multiples of that number, for example.

Read more about Sass here.

OOCSS in Practice

When writing CSS for my personal website recently, I was amazed to see how soothing it was to simply focus on one component at a time.

For example, my design used a base .module block for a lot of the elements, which I then extended with variations such as .module--billboard, .module--callout, and .module--card. Since I made the design myself, I was mostly just playing around with different variations until something struck my fancy. Always while constructing, I tried to abstract all the common styles into the base class and then put different behaviors into the extensions.

My base module was defined as this:

.module {
  padding: $default-spacing;
  .module__head {
    padding: ($default-spacing / 2) 0;

And as an example, my .module--card looked like this:

.module--card {
  .module__body {
     @include purple-gradient;
     .image-frame__image {
       @include ease-transition;
       opacity: 1;
          &:hover {
            opacity: 0.5;
  .module__footer {
      background-color: $white;
      position: relative;
      top: ($default-spacing / -2);
      padding: $default-spacing;
      -webkit-box-shadow: 0 10px 6px -6px #777;
      -moz-box-shadow: 0 10px 6px -6px #777;
      box-shadow: 0 10px 6px -6px #777;

I could probably clean up my modules a bit more, but the idea is that there is a base module, which I then extend with additional styles.

The markup on the page would look something like this:

<div class="module module--card">
  <div class="module__body">
    Some body content goes here.
  <div class="module__footer">
    Some footer content goes here.

And in this manner, but separating skin from structure and content from container, these styles become portable across my site. You could imagine that on another page, since there are no references to ids, I could simply apply the classes module and module--card to my divs and instantly style them consistently. It's brilliant!

See the full code here.

There are some arguments against writing a whole framework for small sites, but I think for the most part it's worth it. Just thinking about CSS in this way makes it more maintainable and extensible. Plus, it makes me feel better that I'm writing something that I can reuse in the future. It's cleaner and way more scalable, and yeah, it's just plain awesome.

Big shoutout to my co-worker Kate Travers for teaching all of us on the Labs team at Flatiron School how to write OOCSS and for enforcing conventions on our framework for She's the best.