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?

I’m not lying. It’s really blue.

You can use any permutation of blue green red – the text is still blue. Try it! 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 className={`italic blue ${className}`}>{children}</p>
); // default P

// ...

const Error = ({ errorText }) => (
    <p className="red">Red error!</p>
); // doesn't become red

The generic P component returns a <p> element with an italicand 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.

What?!

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?

Editorial Note: If you'd like to play around with this in Codepen, here's a link to the original article.