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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.

All packages are different, so refer to their docs for how they work.

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HTML 

              
                <!-- https://creativecommons.org/licenses/by-sa/4.0/ -->

<header>

    <h2>Header</h2>

    <nav>
        <ul>
            <li><a href="#">Nav 1</a></li>
            <li><a href="#">Nav 2</a></li>
            <li><a href="#">Nav 3</a></li>
            <li><a href="#">Nav 4</a></li>
            <li><a href="#">Nav 5</a></li>
            <li><a href="#">Nav 6</a></li>
            <li><a href="#">Nav 7</a></li>
            <li><a href="#">Nav 8</a></li>
            <li><a href="#">Nav 9</a></li>
            <li><a href="#">Nav 10</a></li>
        </ul>
    </nav>

</header>
<main>

    <h1>Scroll-Margin Prototype (not fully supported)</h1>

    <p>To see the scroll-margin in action: TAB down to the bottom of the page, and then start to SHIFT+TAB back up again. When keyboard focus reaches the bottom of the header, the scroll position of focused elements is constrained to the bottom of the header (where supported).</p>

    <ul>
        <li><a href="#">Link 1</a></li>
        <li><a href="#">Link 2</a></li>
        <li><a href="#">Link 3</a></li>
        <li><a href="#">Link 4</a></li>
        <li><a href="#">Link 5</a></li>
        <li><a href="#">Link 6</a></li>
        <li><a href="#">Link 7</a></li>
        <li><a href="#">Link 8</a></li>
        <li><a href="#">Link 9</a></li>
        <li><a href="#">Link 10</a></li>
        <li><a href="#">Link 11</a></li>
        <li><a href="#">Link 12</a></li>
        <li><a href="#">Link 13</a></li>
        <li><a href="#">Link 14</a></li>
        <li><a href="#">Link 15</a></li>
        <li><a href="#">Link 16</a></li>
        <li><a href="#">Link 17</a></li>
        <li><a href="#">Link 18</a></li>
        <li><a href="#">Link 19</a></li>
        <li><a href="#">Link 20</a></li>
        <li><a href="#">Link 21</a></li>
        <li><a href="#">Link 22</a></li>
        <li><a href="#">Link 23</a></li>
        <li><a href="#">Link 24</a></li>
        <li><a href="#">Link 25</a></li>
        <li><a href="#">Link 26</a></li>
        <li><a href="#">Link 27</a></li>
        <li><a href="#">Link 28</a></li>
        <li><a href="#">Link 29</a></li>
        <li><a href="#">Link 30</a></li>
    </ul>

</main>
!

CSS 

              
                :root {
    font-family: sans-serif;
    font-size: 1.2em;
}
html, body {
    background: #fff;
    margin: 0;
    color: #444;
}
ul, li {
    list-style: none;
    margin: 0;
    padding: 0;
}
a:focus {
    outline: 2px solid #05f;
    outline-offset: 2px;
}

header {
    background: #fec;
    border: 2px solid #f60;
    box-sizing: border-box;
    left: 0;
    padding: 1rem;
    position: sticky;
    top: 0;
    width: 100%;
}
header h2 {
    font-weight: normal;
    margin: 0 0 1rem 0;
}
header nav,
header nav ul,
header nav li {
    display: flex;
    flex-wrap: wrap;
}
header nav a {
    background: #fff;
    border: 2px solid #f60;
    color: #05f;
    display: block;
    margin: 0 2px 2px 0;
    padding: 0.4rem 0.7rem;
    position: relative;
    z-index: 0;
}
header nav a:focus {
    z-index: 1;
}

main {
    padding: 2rem;
}
main h1 {
    font-weight: normal;
    margin: 0 0 1rem 0;
}
main p {
    line-height: 1.5;
    margin: 0 0 1rem 0;
}
main a {
    background: #eee;
    border: 2px solid #666;
    color: #05f;
    display: inline-block;
    margin: 0 0 6px 0;
    min-width: 4rem;
    padding: 0.4rem 0.7rem;
}
!

JS 

              
                window.addEventListener('DOMContentLoaded', () => {

    if(!!!('ResizeObserver' in window)) { return; }

    const header = document.querySelector('header');
    const style = document.head.appendChild(document.createElement('style'));
    const stylesheet = style.sheet;

    const applyScrollMargin = () => {

        while(stylesheet.cssRules.length) {
            stylesheet.deleteRule(0);
        }

        let height = header.offsetHeight + 6;
        stylesheet.insertRule(`main *:focus { scroll-margin-top:${height}px }`);

        //this catches changes in header height while an element is already focused
        let focused = document.activeElement || document.body;
        if(focused !== document.body && !header.contains(focused)) {
            if(focused.getBoundingClientRect().top < height) {
                focused.scrollIntoView();
            }
        }
    };

    (new ResizeObserver(applyScrollMargin)).observe(header);

});
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999px

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