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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

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).

Add External Stylesheets/Pens

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You can apply CSS to your Pen from any stylesheet on the web. Just put a URL to it here and we'll apply it, in the order you have them, before the CSS in the Pen itself.

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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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You can apply a script from anywhere on the web to your Pen. Just put a URL to it here and we'll add it, in the order you have them, before the JavaScript in the Pen itself.

If the script you link to has the file extension of a preprocessor, we'll attempt to process it before applying.

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Packages

Add Packages

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 

              
                <h1>Scroll Detection with<br>CSS Scroll-Driven Animations<br><em>(Space Toggle)</em></h1>
<main>
	<div>
		<div class="container">
			<p>CSS, short for Cascading Style Sheets, is a programming language that is used to describe the presentation or visual layout of a document written in HTML. It acts as a styling tool and enables web developers to control the appearance of their websites or web pages by specifying how different elements should be displayed on the screen. CSS allows designers to define various aspects such as font styles, colors, spacing between elements, background images, and much more.</p>
			<p>The term "cascading" in CSS refers to its ability to apply multiple stylesheets simultaneously and prioritize them based on their importance or specificity. This feature allows developers to create consistent designs across multiple web pages by defining common styles in an external stylesheet which can then be linked to all relevant HTML documents. Additionally, CSS makes it easy to update the design of an entire website by simply modifying one central stylesheet rather than having to manually edit each individual webpage.</p>
			<p>CSS can be applied directly within an HTML document using inline stylesheets or internally through embedded stylesheets within the head section of a page. However, it is considered best practice and more efficient from both maintenance and performance perspectives to use external style sheets stored separately in .css files which are then referenced within HTML documents.</p>
			<p>One important concept in CSS is selectors. Selectors allow you to target specific elements within your HTML markup so that you can apply different styles only where needed. Selectors can target tags (e.g., p, div), classes (e.g., .header), IDs (e.g., #main), attributes (e.g., [type="submit"]), pseudo-classes (e.g., :hover) and many other properties of DOM elements.</p>
			<p>Overall, CSS plays a crucial role in modern web development as it provides great flexibility and control over how content looks on websites. By separating style from structure with this powerful language, developers are able to create visually appealing websites while maintaining clean code that adheres to best practices for web accessibility and user experience.</p>
		</div>
		<p>&uarr; This container can scroll</p>
	</div>

	<div>
		<div class="container">
			<p>CSS, short for Cascading Style Sheets, is a programming language that is used to describe the presentation or visual layout of a document written in HTML.</p>
		</div>
		<p>&uarr; This container can’t scroll</p>
	</div>
</main>

<footer>
	<p>Demo for <a href="https://brm.us/css-can-scroll" target="_top">https://brm.us/css-can-scroll</a></p>
</footer>
!

CSS 

              
                .container {
	height: 250px;
	width: 250px;
  overflow-y: auto;
	
	--can-scroll: initial; /* initial = false */
  animation: detect-scroll;
  animation-timeline: scroll(self);
	
	--color-if-can-scroll: var(--can-scroll) lime;
	--color-if-cant-scroll: red;
	outline: 10px dotted var(--color-if-can-scroll, var(--color-if-cant-scroll));
}

@keyframes detect-scroll {
  from, to {
    --can-scroll: ; /* space = true */
  }
}



* {
  box-sizing: border-box;
}

body {
  font-size: 17px;
  font-family: -system-ui, sans-serif;
  line-height: 1.5;
  letter-space: -0.01em;
  margin: 0;
  padding: 2em;
  color: #444;
  background: #cfcfcf;
  -webkit-font-smoothing: antialiased;
  
  display: grid;
  place-content: safe center;
  height: 100%;
  gap: 2rem;
}

h1 {
	margin: 0;
	padding: 0;
	text-align: center;
}

main {
	display: flex;
  gap: 2rem;
	margin: 0 auto;
	text-align: center;
}

html, body {
  height: 100%;
}

.container {
  position: relative;
  background: #fff;
  padding: 1em;
  margin: auto;
  border-radius: .3em;
  box-shadow: 0 4px 24px rgba(0, 0, 0, 0.1);
	text-align: left;
}

.container p {
	margin-bottom: 1em;
}

.container > *:first-child {
	margin-top: 0;
}
.container > *:last-child {
	margin-bottom: 0;
}

footer {
  text-align: center;
  font-style: italic;
  font-size: 80%;
}
!

JS
!
999px

Console