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HTML

About HTML Preprocessors

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

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

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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HTML 

              
                <header>
      <button aria-pressed="true">
        <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="currentColor" class="w-6 h-6">
          <path fill-rule="evenodd" d="M9.528 1.718a.75.75 0 01.162.819A8.97 8.97 0 009 6a9 9 0 009 9 8.97 8.97 0 003.463-.69.75.75 0 01.981.98 10.503 10.503 0 01-9.694 6.46c-5.799 0-10.5-4.701-10.5-10.5 0-4.368 2.667-8.112 6.46-9.694a.75.75 0 01.818.162z" clip-rule="evenodd" />
          <path d="M12 2.25a.75.75 0 01.75.75v2.25a.75.75 0 01-1.5 0V3a.75.75 0 01.75-.75zM7.5 12a4.5 4.5 0 119 0 4.5 4.5 0 01-9 0zM18.894 6.166a.75.75 0 00-1.06-1.06l-1.591 1.59a.75.75 0 101.06 1.061l1.591-1.59zM21.75 12a.75.75 0 01-.75.75h-2.25a.75.75 0 010-1.5H21a.75.75 0 01.75.75zM17.834 18.894a.75.75 0 001.06-1.06l-1.59-1.591a.75.75 0 10-1.061 1.06l1.59 1.591zM12 18a.75.75 0 01.75.75V21a.75.75 0 01-1.5 0v-2.25A.75.75 0 0112 18zM7.758 17.303a.75.75 0 00-1.061-1.06l-1.591 1.59a.75.75 0 001.06 1.061l1.591-1.59zM6 12a.75.75 0 01-.75.75H3a.75.75 0 010-1.5h2.25A.75.75 0 016 12zM6.697 7.757a.75.75 0 001.06-1.06l-1.59-1.591a.75.75 0 00-1.061 1.06l1.59 1.591z" />
        </svg>
      </button> 
      <svg viewBox="0 0 76 65" fill="none" xmlns="http://www.w3.org/2000/svg">
        <path d="M37.5274 0L75.0548 65H0L37.5274 0Z" fill="#000000" />
      </svg>
    </header>
    <main>
      <section>
        <h1>Next.js on Vercel</h1>
        <span
          >Vercel is the native Next.js platform, designed to enhance the
          Next.js experience.</span
        >
        <p>
          Next.js is a fullstack React framework for the web, maintained by
          Vercel.
        </p>
        <p>
          While Next.js works when self-hosting, deploying to Vercel is
          zero-configuration and provides additional enhancements for
          <mark data-author="JT"
            ><span
              >scalability, availability, and performance globally.</span
            ></mark
          >
        </p>
      </section>
      <section>
        <h2 id="getting-started">Getting started</h2>
        <p>There are multiple ways to get started with Next.js on Vercel:</p>
        <ul>
          <li>
            If you already have a project with Next.js, install Vercel CLI and
            run the vercel command from your project&#39;s root directory
          </li>
          <li>
            Clone one of our Next.js example repos to your favorite git provider
            and deploy it on Vercel with the button below:
          </li>
          <li>Or, choose a template from Vercel&#39;s marketplace:</li>
        </ul>
        <p>
          <mark data-author="RH"
            ><span
              >Vercel deployments can integrate with your git provider to
              generate preview URLs for each pull request you make</span
            ></mark
          >
          to your Next.js project.
        </p>
      </section>
      <section>
        <h2 id="incremental-static-regeneration">
          Incremental Static Regeneration
        </h2>
        <p>
          Incremental Static Regeneration (ISR) allows you to
          <mark data-author="AB" style="--hue: 280"
            ><span
              >create or update content without redeploying your site</span
            ></mark
          >. ISR has three main benefits for developers: better performance,
          improved security, and faster build times.
        </p>
        <p>
          When self-hosting, (ISR) is limited to a single region workload.
          Statically generated pages are not distributed closer to visitors by
          default, without additional configuration or vendoring of a CDN. By
          default, self-hosted ISR does not persist generated pages to durable
          storage. Instead, these files are located in the Next.js cache (which
          expires).
        </p>
        <p>
          <mark data-author="GR" style="--hue: 30"
            ><span
              >To enable ISR with Next.js in the app router, add an options
              object with a revalidate property to your fetch requests</span
            ></mark
          >.
        </p>
        <p>To summarize, using ISR with Next.js on Vercel:</p>
        <ul>
          <li>Better performance with our global Edge Network</li>
          <li>
            Zero-downtime rollouts to previously statically generated pages
          </li>
          <li>
            Framework-aware infrastructure enables global content updates in
            300ms
          </li>
          <li>
            Generated pages are both cached and persisted to durable storage
          </li>
        </ul>
      </section>
      <section>
        <h2 id="server-side-rendering-ssr-">Server-Side Rendering (SSR)</h2>
        <p>
          Server-Side Rendering (SSR) allows you to render pages dynamically on
          the server. This is useful for pages where the rendered data needs to
          be unique on every request. For example, checking authentication or
          looking at the location of an incoming request.
        </p>
        <p>
          On Vercel,
          <mark data-author="LR" style="--hue: 220"
            ><span
              >you can server-render Next.js applications in either the Node.js
              runtime (default) with Serverless Functions or the Edge runtime
              with Edge Functions</span
            ></mark
          >. This allows you to pick the best rendering strategy on a per-page
          basis.
        </p>
        <p>To summarize, SSR with Next.js on Vercel:</p>
        <ul>
          <li>Scales to zero when not in use</li>
          <li>Scales automatically with traffic increases</li>
          <li>
            Has zero-configuration support for Cache-Control headers, including
            stale-while-revalidate
          </li>
          <li>
            Framework-aware infrastructure enables switching rendering between
            Edge/Node.js runtimes
          </li>
        </ul>
      </section>
      <section>
        <h2 id="image-optimization">Image Optimization</h2>
        <p>
          Image Optimization helps you achieve faster page loads by reducing the
          size of images and using modern image formats.
        </p>
        <p>
          When deploying to Vercel,
          <mark data-author="MN" style="--hue: 140"><span>images are automatically optimized on demand</span></mark
          >, keeping your build times fast while improving your page load
          performance and Core Web Vitals.
        </p>
        <p>
          When self-hosting, Image Optimization uses the default Next.js server
          for optimization. This server manages the rendering of pages and
          serving of static files.
        </p>
        <p>
          To use Image Optimization with Next.js on Vercel,
          <mark data-author="IR" style="--hue: 10"
            ><span
              >import the next/image component into the component you&#39;d like
              to add an image to</span
            ></mark
          >.
        </p>
        <p>To summarize, using Image Optimization with Next.js on Vercel:</p>
        <ul>
          <li>Zero-configuration Image Optimization when using next/image</li>
          <li>Helps your team ensure great performance by default</li>
          <li>
            <mark data-author="SH" style="--hue: 190"
              ><span
                >Keeps your builds fast by optimizing images on-demand</span
              ></mark
            >
          </li>
          <li>Requires No additional services needed to procure or set up</li>
        </ul>
      </section>
      <hr />
      <p>
        <a href="https://vercel.com/docs/frameworks/nextjs"
          >Check out the Vercel Docs for more.</a
        >
      </p>
    </main>
!

CSS 

              
                @font-face {
  font-family: "Geist Sans";
  src: url("https://assets.codepen.io/605876/GeistVF.ttf") format("truetype");
}

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

body {
	min-height: 100vh;
	font-weight: 80;
	padding: 0 4rem 0 1rem;
	background: hsl(0 0% 98%);
	color: hsl(0 0% 6%);
	font-family:  "Geist Sans", "SF Pro Text", "SF Pro Icons", "AOS Icons", "Helvetica Neue", Helvetica, Arial, sans-serif, system-ui;
}

body::after {
	--size: 80px;
	--thickness: 2px;
	content: "";
	position: fixed;
	height: 100vh;
	width: 100vw;
	pointer-events: none;
	background:
		linear-gradient(transparent 0 calc(var(--size) - var(--thickness)), currentColor calc(var(--size) - var(--thickness)) var(--size)) 50% 50% / var(--size) var(--size),
		linear-gradient(90deg, transparent 0 calc(var(--size) - var(--thickness)), currentColor calc(var(--size) - var(--thickness)) var(--size)) 50% 50% / var(--size) var(--size);
	mask: linear-gradient(-35deg, transparent 50%, white);
	top: 0;
	left: 0;
	z-index: -1;
	opacity: 0.25;
}

.dark body {
	background: hsl(0 0% 6%);
	color: hsl(0 0% 96%);
}

mark {
	color: hsl(0 0% 6%);
}

.dark mark {
	color: hsl(0 0% 98%);
}

section {
	margin-bottom: 2rem;
}

.dark header svg path {
	fill: white;
}

* {
  box-sizing: border-box;
}

:where(h1, h2) {
	font-weight: 120;
}

code {
	max-width: 100%;
	overflow: auto;
}

header {
	height: 50vmin;
	min-height: 200px;
	width: 40ch;
  max-width: 100%;
  margin: 0 auto;
  font-size: 1.25rem;
  display: grid;
  align-content: center;
}

header svg {
	width: 35%;
	filter: drop-shadow(0 0 20px hsl(0 0% 50% / 0.75));
}

main {
	padding: 0 1rem;
  width: 40ch;
  max-width: 100%;
  margin: 0 auto;
  font-size: 1.25rem;
  padding: 0 0 50vmin 0;
}

button {
	position: fixed;
	top: 1rem;
	right: 1rem;
	width: 48px;
	aspect-ratio: 1;
	padding: 0;
	border-radius: 12px;
	border: 0;
	background: transparent;
	display: grid;
	place-items: center;
	cursor: pointer;
}

[aria-pressed=true] svg path:last-of-type,
[aria-pressed=false] svg path:first-of-type {
	display: none;
}

button svg {
	width: 65%;
}

mark {
	--lightness: 80%;
  --highlighted: 1;
  --highlight: hsl(var(--hue, 45) 80% var(--lightness));
  background: transparent;
}

@supports (animation-timeline: view()) {
  mark {
    --highlighted: 0;
    animation: highlight steps(1) both;
    animation-timeline: view();
    animation-range: entry 100% cover 10%;
  }
}

.dark mark {
	--lightness: 35%;
}

mark span {
  background: linear-gradient(120deg, var(--highlight, lightblue) 50%, transparent 50%) 110% 0 / 200% 100% no-repeat;
  background-position: calc((1 - var(--highlighted)) * 110%) 0;
  transition: background-position 1s;
}
p, li {
  position: relative;
}

a {
	color: hsl(212 100% 50%);
	text-decoration: none;
}

a:is(:hover, :focus-visible) {
	text-decoration: underline;
	text-underline-offset: 6px;
}

hr {
	margin: 2rem auto;
}

hr + p {
	text-align: center;
}

mark::after {
  content: attr(data-author);
  display: grid;
  place-items: center;
  font-variant-numeric: tabular-nums;
  font-weight: bold;
  position: absolute;
  width: 32px;
  aspect-ratio: 1;
  border-radius: 12px;
  background: var(--highlight);
  font-weight: 80;
  top: 0;
  left: 100%;
  translate: 50% 0;
  font-size: 0.875rem;
  scale: var(--highlighted);
  transition: scale 0.2s;
}

@keyframes highlight {
  to {
    --highlighted: 1;
  }
}

::view-transition-new(root) {
	animation: grow 1s;
}
::view-transition-old(root) {
	animation: none;
}

@keyframes grow {
	0% {
		clip-path: polygon(50vmax 50vmax, 50vmax 50vmax, 50vmax 50vmax);
	}
	100% {
		clip-path: polygon(50vmax -100vmax, -100vmax 200vmax, 200vmax 200vmax);
	}
}
!

JS 

              
                const TOGGLE = document.querySelector("button");

const UPDATE = () => {
  const DARK = TOGGLE.matches("[aria-pressed=true]");
  TOGGLE.setAttribute("aria-pressed", DARK ? false : true);
  document.documentElement.className = DARK ? "dark" : "";
};

const TOGGLE_THEME = () => {
  if (!document.startViewTransition) return UPDATE();
  document.startViewTransition(() => UPDATE());
};

TOGGLE.addEventListener("click", TOGGLE_THEME);

if (!CSS.supports("animation-timeline: view()")) {
  const MARKS = document.querySelectorAll("mark");
  const OPTS = {
    threshold: 1.0
  };
  const HANDLE = (entries) => {
    entries.forEach((entry) => {
      entry.target.style.setProperty(
        "--highlighted",
        entry.isIntersecting ? 1 : 0
      );
    });
  };
  const OBSERVER = new IntersectionObserver(HANDLE, OPTS);
  MARKS.forEach((M) => OBSERVER.observe(M));
}
!
999px

Console