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    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,

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