Skip to content
Swizec Teller - a geek with a hatswizec.com

CSS classes don't work the way you think they work

<p class="blue green red">This is some text</p>

Which color is this text going to be?

No cheating, give it a think ??

.

.

.

.

Blue. The answer is blue. ?

Look. I’m not lying. It’s really blue.

You can use any permutation of blue green red – the text is still blue. Try it! Edit the codepen. Play around :)

Did you find the pattern?

CSS classes apply in the order in which they are defined, not the order in which they are invoked. This is not intuitive.

Look: if you switch around the CSS rules, the text becomes red.

Same HTML, same CSS classes, different order of definitions. Try it; change the code.

Maybe this is obvious to everyone but me, but I spent an embarrassing amount of time yesterday and today debugging some React components. It hits you when common components have default styling, and you want to override it in a specific instance.

const P = ({ className, children }) => (
<p class="{`italic" blue="" ${classname}`}="">{children}</p>
); // default P
// ...
const Error = ({ errorText }) => (
<p class="red">Red error!</p>
); // doesn't become red

The generic P component returns a <p> element with an italic and a blue class. You can expect text to be italic and blue by default.

Please don’t do that in real life. This is just an example.

It takes a className prop so you can extend classes used.

But when you use the Error component, which produces <p class="italic blue red">Red error!</p>, it’s not red. It’s blue because your CSS defines .red first and .blue second.

?

There is no workaround. This is expected behavior. The relevant part of W3C spec makes no mention of HTML attribute ordering.

How did I go 15 years without ever noticing? ?

Did you enjoy this article?

Published on October 21st, 2016 in Front End, Technical

Learned something new?
Want to become a high value JavaScript expert?

Here's how it works 👇

Leave your email and I'll send you an Interactive Modern JavaScript Cheatsheet 📖right away. After that you'll get thoughtfully written emails every week about React, JavaScript, and your career. Lessons learned over my 20 years in the industry working with companies ranging from tiny startups to Fortune5 behemoths.

Start with an interactive cheatsheet 📖

Then get thoughtful letters 💌 on mindsets, tactics, and technical skills for your career.

"Man, love your simple writing! Yours is the only email I open from marketers and only blog that I give a fuck to read & scroll till the end. And wow always take away lessons with me. Inspiring! And very relatable. 👌"

~ Ashish Kumar

Join over 10,000 engineers just like you already improving their careers with my letters, workshops, courses, and talks. ✌️

Have a burning question that you think I can answer? I don't have all of the answers, but I have some! Hit me up on twitter or book a 30min ama for in-depth help.

Ready to Stop copy pasting D3 examples and create data visualizations of your own?  Learn how to build scalable dataviz components your whole team can understand with React for Data Visualization

Curious about Serverless and the modern backend? Check out Serverless Handbook, modern backend for the frontend engineer.

Ready to learn how it all fits together and build a modern webapp from scratch? Learn how to launch a webapp and make your first 💰 on the side with ServerlessReact.Dev

Want to brush up on your modern JavaScript syntax? Check out my interactive cheatsheet: es6cheatsheet.com

By the way, just in case no one has told you it yet today: I love and appreciate you for who you are ❤️