Building A Relationship Between CSS & JavaScript

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Tim Wright is the Senior User Experience Designer and Developer at Fresh Tilled Soil, a UX design agency out of Boston, MA. He is a frequent speaker, blogger, … More about Tim ↬

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Even though we keep JavaScript, CSS and HTML in different files, the concepts behind progressive enhancement are getting all knotted up with every jQuery plugin we use and with every weird technique that crops up. In this post, Tim Wright offers a little perspective as to where we are and how we can realign our goals.

jQuery, Prototype, Node.js, Backbone.js, Mustache and thousands of JavaScript microlibraries all combine into a single undeniable fact: JavaScript is popular. It’s so popular, in fact, that we often find ourselves using it in places where another solution might be better in the long run.

Even though we keep JavaScript, CSS and HTML in different files, the concepts behind progressive enhancement are getting all knotted up with every jQuery plugin we use and with every weird technique that crops up. Because JavaScript is so powerful, there are a lot of overlaps in capability between JavaScript and HTML (building document structure) and JavaScript and CSS (injecting style information).

I’m not here to pick on any JavaScript library, bootstrap or boilerplate; I’m just here to offer a little perspective as to where we are and how we can realign our goals.

Relationship Image
Image Credit: opensourceway.

Keeping CSS Out Of Your JavaScript

CSS can hook into HTML with a variety of different selectors; this isn’t anything new. By using IDs, classes or any attribute you can think of (even custom attributes), you have easy access to style an element. You can also do this with a slew of JavaScript methods, and honestly, it’s the same basic process with a different syntax (one of my JavaScript ah-ha moments). Being able to natively access HTML from JavaScript and from CSS is one of the reasons progressive enhancement has been such a successful development model. It allows a point of reference to guide us and to serve as a reminder as we develop a project, so we don’t “cross the streams”.

But, as you move forward with JavaScript and build applications with highly interactive elements, it gets harder to not only keep HTML out of your JavaScript, but also to catch yourself before injecting style information into a document. Of course, the case for not injecting style with JavaScript certainly isn’t a binary one (yes/no, true/false, 0/1); there are plenty of cases where you might need to apply styles progressively, for example, in a drag and drop interface where positioning information needs to be constantly updated based on cursor (or finger) position.

But generally speaking, you can safely house all the style information you need within CSS and reference styles as reusable classes. This is a much more flexible model than sprinkling CSS throughout a JavaScript file, and it very closely compares to the model of adding style information into your HTML. We follow this model when it’s only HTML and CSS, but for some reason it has a tendency to fall apart once JavaScript gets added into the mix. It’s certainly something we need to keep an eye on.

A lot of front-end developers take real pride in having clean HTML. It’s easy to work with, and to certain super-geeks it can even be artful. It’s great to have clean, static HTML, but what good is that if your generated HTML is riddled with injected style and non-semantic markup? By “generated HTML,” I’m referencing how the HTML looks after it’s been consumed and barfed back up after being passed around all those plugins and extra JavaScript. If step one to having clean HTML and separated progressive enhancement layers is to not use a style attribute, I’d have to say that step two is to avoid writing JavaScript that injects a style attribute for you.

Cleaning Up Your HTML

We can probably all agree that blindly using a technology is a terrible idea, and I think we’re at a point with jQuery where we are, indeed, blindly using a lot of the features without fully understanding what’s going on under the hood. The example I lean on pretty heavily for keeping CSS out of my JavaScript is the behavior of jQuery’s hide() method. Based on the principles of progressive enhancement, you wouldn’t code something with inline CSS like this:

<div class="content-area" style="display:none;"></div>

We don’t do that because a screen reader won’t pick up an element if the style is set to display:none, and it also muddies up the HTML with unnecessary presentational information. When you use a jQuery method like hide(), that’s exactly what it does: it will set a style attribute on the target area and add a display property of none. It’s very easy to implement, but not very good for accessibility. It also violates the principles of progressive enhancement when you inject style into the document like that (we’re all sorts of messed up, huh?). It’s not uncommon for this method to be used within a tabbing interface to hide content. The result is that the content is nonexistent to a screen reader. Once we realize that adding style from JavaScript isn’t ideal in most cases, we can move it into the CSS and reference it as a class:


.hide {
   display: none;



We still have to address the accessibility problem of hiding content with display:none, but since we’re not using a built-in jQuery method anymore, we can control exactly how content gets hidden (whichever accessible method you prefer is probably fine). For example we could do something like:


.hide {
   position: absolute;
   top: -9999px;
   left: -9999px;

.remove {
   display: none;

In the above example, you can see that even though both classes result in content being removed from view, they function very differently from an accessibility standpoint. Looking at the code like this makes it clear that we really are dealing with style information that belongs in a CSS file. Using utility classes in this way can not only help your JavaScript slim down, but also have double usage in an Object Oriented CSS (OOCSS) development model. This is truly a way to not repeat yourself (Don’t Repeat Yourself, or DRY) within CSS, but also across a whole project, creating a more holistic approach to front-end development. Personally, I see a lot of benefit in controlling your behaviors this way, but some people have also called me a control-freak in the past.

Web and Team Environments

This is a way we can start opening up lines of communication between CSS and JavaScript and lean on the strengths of each language without overdoing it. Creating a developmental balance on the front end is very important, because the environment is so fragile and we can’t control it like we can on the back end with a server. If a user’s browser is old and slow, most of the time you can’t sit down and upgrade it (aside: I do have my grandmother using Chrome); all you can do is embrace the environmental chaos, build for the best and plan for the worst.

Some people have argued with me in the past that this style of development, where you’re referencing CSS classes in JavaScript, doesn’t work well in team development environments because the CSS is usually built by the time you’re diving into the JavaScript, which can cause these classes to get lost in the mix and create a lot of inconsistency in the code (the opposite of DRY). To those people I say: poke your head over the cube wall, open AIM, GTalk or Skype, and communicate to the rest of the team that these classes exist specifically to be used with JavaScript. I know the concept of developers communicating outside of GIT commit messages seems like madness, but it’ll be okay, I promise.

Using Behavioral CSS With JavaScript Fallbacks

Using these CSS objects as hooks for JavaScript can go far beyond simple hiding and showing of content into an area of behavioral CSS, transitions, animations and transforms that are often done with JavaScript animations. With that in mind, lets take a look at a common interaction model of fading out a div on click, and see how it would be set up with this development model, while providing the proper fallbacks for browsers that might not support the CSS transition we’re going to use.

For this example we’ll be using:

First, let’s set up our body element:

    <button type="button">Run Transition</button>
    <div id="cube"></div><!--/#cube-->

From there we’ll need to set up the CSS:

#cube {
   height: 200px;
   width: 200px;
   background: orange;
   -webkit-transition: opacity linear .5s;
      -moz-transition: opacity linear .5s;
        -o-transition: opacity linear .5s;
           transition: opacity linear .5s;

.fade-out {
   opacity: 0;

Before we add on the JavaScript layer, lets take a moment and talk about the flow of what’s going to happen:

  1. Use Modernizr to check for CSS Transition support
  2. If Yes
    1. Set up a click event on the button to add a “fade-out” class to #cube
    2. Add another event listener to catch when the transition is finished so we can time the execution of a function that will remove #cube from the DOM.
  3. If No
    1. Set up a click even on the button to use jQuery’s animate() method to manually fade #cube out.
    2. Execute a callback function to remove #cube from the DOM.

This process will introduce a new event called transitionend, which will execute at the end of a CSS transition. It’s amazing, FYI. There is also a companion event called animationend, which will execute at the end of a CSS animation for more complex interactions.

First thing we need to do is set up our variables in the JavaScript:

(function () {

   // set up your variables
   var elem = document.getElementById('cube'),
       button = document.getElementById('do-it'),
       transitionTimingFunction = 'linear',
       transitionDuration = 500,

   // set up the syntax of the transitionend event with proper vendor prefixes
   if ($.browser.webkit) {
       transitionend = 'webkitTransitionEnd'; // safari & chrome
   } else if ($.browser.mozilla) {
       transitionend = 'transitionend'; // firefox
   } else if ($.browser.opera) {
       transitionend = 'oTransitionEnd'; // opera
   } else {
       transitionend = 'transitionend'; // best guess at the default?

   //... rest of the code goes here.

})(); // end wrapping function

You might notice that our new transitionend event needs a vendor prefix; we’re doing a little browser detection to take care of that. Normally you might detect for the vendor prefix and add it onto the event name, but in this instance the cases for the syntaxes are a little different, so we need to get the whole name of the event for each prefix.

In the next step we’ll use Modernizr to detect support, and add our event listeners to each case (all of this stuff gets added inside the wrapping function):

// detect for css transition support with Modernizr
if(Modernizr.csstransitions) {

    // add our class on click
    $(button).on('click', function () {

    // simulate a callback function with an event listener
    elem.addEventListener(transitionend, function () {
    }, false);

} else {

   // set up a normal click/animate listener for unsupported browsers
   $(button).on('click', function () {

           'opacity' : '0'
       }, transitionDuration, transitionTimingFunction, function () {

   }); // end click event

} // end support check

Finally, we need to define a shared function between the two actions (DRY) which executes after the transition (or animation) is complete. For the sake of this demonstration we can just call it theCallbackFunction() (even though it’s not technically a callback function). It will remove an element from the DOM and spit out a message in the console letting us know that it worked.

// define your callback function, what happens after the transition/animation
function theCallbackFunction (elem) {

   'use strict';

   // remove the element from the DOM

   // log out that the transition is done
   console.log('the transition is complete');


In the browser, this should work the same way in IE 7 (on the low end) as it does in mobile Safari or Chrome for Mobile (on the high end). The only difference is under the hood; the experience never changes for the user. This is a way you can use cutting-edge techniques without sacrificing the degraded user experience. It also keeps CSS out of your JavaScript, which was really our goal the whole time.

The Moral Of The Story

You might be asking yourself why we should even bother going through all this work. We wrote about 60 lines of JavaScript to accomplish the same design aesthetic that could be created with eight lines of jQuery. Well, no one ever said that keeping clean code and sticking to progressive enhancement was the easiest thing to do. In fact, it’s a lot easier to ignore it entirely. But as responsible developers, it’s our duty to build applications in a way that is accessible and easily scales to the future. If you want to go that extra mile and create a seamless user experience as I do, then it’s well worth the extra time it takes to dot all the i’s and cross all the t’s in a project to create an overall experience that will gracefully degrade and progressively enhance.

Using this model also lets us lean heavily on CSS for its strengths, like responsive design and using breakpoints to redefine your interaction at the various screen sizes. It also helps if you’re specifically targeting a device with a constrained bandwidth, because, as we all know, CSS is much lighter than JavaScript in both download and execution time. Being able to offload some of the weight JavaScript carries onto CSS is a great benefit.

In production, we are currently using CSS animations and transitions for micro interactions like hover effects and maybe a spinning graphic or a pulsating knot. We’ve come to a point where CSS is a pretty powerful language that performs very well in the browser and it’s okay to use it more heavily for those macro interactions that are typically built using JavaScript. If you’re looking for a lightweight and consistent experience that’s relatively easy to maintain while allowing you to use the latest and greatest browser features — it’s probably time to start mending fences and build strength back into the relationship between CSS and JavaScript. As a great man once said, “The key to writing great JavaScript is knowing when to use CSS instead.” (It was me… I said that.)

Further Reading

Smashing Editorial (cp, mrn)