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HTML

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HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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Adding Classes

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About the <head>

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. If you need things in the <head> of the document, put that code here.

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Insecure Resource

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CSS

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

We offer two popular choices: Autoprefixer (which processes your CSS server-side) and -prefix-free (which applies prefixes via a script, client-side).

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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Packages

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Search for and use JavaScript packages from npm here. By selecting a package, an import statement will be added to the top of the JavaScript editor for this package.

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Using packages here is powered by esm.sh, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ESM usage.

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HTML 

              
                <html>
	<head>
		<title>Accordion</title>
	</head>
	<body>
		<div>
    <details open>
        <summary>
            How do you create an accordion?
        </summary>
        <div>
            Easy! As long as you don't have to support IE11 or older browsers you could use <code>&lt;details&gt;</code> and <code>&lt;summary&gt;</code> natively.
        </div>
    </details>
    <details>
        <summary>
            What if I have to support IE11 or older browsers?
        </summary>
        <div>
            No worries. The fallback for these elements is quite good. They will display as open. You won't get the open/close mechanism, but you won't lose any content either.
        </div>
    </details>
    <details>
        <summary>
            What type of content can I have inside one of these?
        </summary>
        <div>
            Almost anything you'd like. The <code>&lt;details&gt;</code> element allows all <a href="https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Content_categories#flow_content" target="_blank">flow content</a>, which is basically everything.
        </div>
    </details>
    <details>
        <summary>
            How does it work?
        </summary>
        <div>
            The <code>&lt;details&gt;</code> element encapsulates the <code>&lt;summary&gt;</code> element. The <code>&lt;summary&gt;</code> becomes the 'label' for the <code>&lt;details&gt;</code> and acts like a button. When clicked, the attribute <code>open</code> is added to the <code>&lt;details&gt;</code> element, making it display. You can therefore style the open and closed states seperately if you'd like.
        </div>
    </details>
</div>
	</body>
</html>
!

CSS 

              
                @import url('https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700;800;900&display=swap');

*, *:after, *:before {
    box-sizing: border-box;
}

:root {
    font-size: 16px;
}

body {
    font-family: "Inter", sans-serif;
    line-height: 1.5;
    min-height: 100vh;
    font-size: 1.25rem;
}

*:focus {
    outline: none;
}

body > div {
    width: 90%;
    max-width: 600px;
    margin-left: auto;
    margin-right: auto;
    margin-top: 5rem;
    margin-bottom: 5rem;
}

details div {
    border-left: 2px solid #000;
    border-right: 2px solid #000;
    border-bottom: 2px solid #000;
    padding: 1.5em;
}

details div > * + * {
    margin-top: 1.5em;
}

details + details {
    margin-top: .5rem;
}

summary {
    list-style: none;
}

summary::-webkit-details-marker {
    display: none;
}

summary {
    border: 2px solid #000;
    padding: .75em 1em;
    cursor: pointer;
    position: relative;
    padding-left: calc(1.75rem + .75rem + .75rem);
}

summary:before {
    position: absolute;
    top: 50%;
    transform: translateY(-50%);
    left: .75rem;
    content: "↓";
    width: 1.75rem;
    height: 1.75rem;
    background-color: #000;
    color: #FFF;
    display: inline-flex;
    justify-content: center;
    align-items: center;
    flex-shrink: 0;
}

details[open] summary {
    background-color: #eee;
}

details[open] summary:before {
    content: "↑";
}

summary:hover {
    background-color: #eee;
}

a {
    color: inherit;
    font-weight: 600;
    text-decoration: none;
    box-shadow: 0 1px 0 0;
}

a:hover {
    box-shadow: 0 3px 0 0;
}

code {
    font-family: monospace;
    font-weight: 600;
}
!

JS
!
999px

Console