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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.

!
Insecure Resource

The resource you are linking to is using the 'http' protocol, which may not work when the browser is using https.

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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Any URL's added here will be added as <script>s in order, and run before the JavaScript in the editor. You can use the URL of any other Pen and it will include the JavaScript from that Pen.

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About External Resources

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.

You can also link to another Pen here, and we'll pull the JavaScript from that Pen and include it. If it's using a matching preprocessor, we'll combine the code before preprocessing, so you can use the linked Pen as a true dependency.

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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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About Packages

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 

              
                <main class="todo-list">
	<h1 class="todo-list__heading">Todo List</h1>
	<ul class="todo-list__list list">
		<li class="list__list-item"><span class="parent">Groceries</span>
			<ul class="list__sublist">
				<li class="list__list-item">3 Tomatoes</li>
				<li class="list__list-item">1 bunch of cherries</li>
				<li class="list__list-item">6 onions</li>
				<li class="list__list-item">3 heads of garlic</li>
				<li class="list__list-item">1 bag spring mix</li>
			</ul>
		</li>
		<li class="list__list-item">Cancel gym membership</li>
		<li class="list__list-item">Clean gutters</li>
		<li class="list__list-item">Take package to the post office</li>
		<li class="list__list-item">Call Avery about Ali's party (afternoon)</li>
		<li class="list__list-item">Sort recycling & put out trash</li>
	</ul>
</main>
!

CSS 

              
                //    Table of Contents:
// 1. Settings
// 2. Tools
// 3. Generic
// 4. Elements
// 5. Objects
// 6. Components

// =====================================================================
// 1. Settings
// 		variable definitions

// =====================================================================
// 2. Tools
// 		mixins and functions

// =====================================================================
// 3. Generic
// 		Normalize taken care of in CodePen settings

// =====================================================================
// 4. Elements
//  	Styling for bare HTML elements (like H1, A, etc.).
body {
	font-family: "Grape Nuts", cursive;
	margin: 1rem;
}

ul,
li {
	list-style: none;
	margin-left: 0;
	padding-left: 0;
}

// =====================================================================
// 5. Objects
//    Class-based selectors which define undecorated design patterns,

.todo-list {
	max-width: 70ch;
	margin: 1rem auto;
	display: flex;
	flex-direction: column;
	align-items: center;

	li {
		margin: 0.5em 0;
		text-transform: capitalize;
	}
}

// =====================================================================
// 6. Components
//    Specific UI components. This is where majority of our work takes place
//    and our UI components are often composed of Objects and Components.

* {
	box-sizing: border-box;
}

.todo-list {
	&__heading {
		text-transform: uppercase;
		text-decoration: underline;
		text-underline-offset: 0.2em;
	}
}

.list {
	font-size: 2rem;

	&__list-item {
		line-height: 1;

		&::before {
			box-sizing: border-box;
			content: "";
			width: 1ch;
			height: 0.8em;
			margin: 0;
			margin-right: 0.5rem;
			border: 1px solid black;
			display: inline-block;
		}

		&--checked {
			&::before {
				content: "X";
			}
		}
	}
}

.list__sublist {
	padding-left: 2rem;
	.list__list-item {
	}
}

.parent {
	text-decoration: underline;
	text-decoration-thickness: 0.1rem;
	text-underline-offset: 0.1em;
}
!

JS 

              
                // toggle checked list styles when list item clicked
const list = document.querySelector("ul");

list.addEventListener("click", (e) => {
	if (e.target.tagName === "LI") {
		e.target.classList.toggle("list__list-item--checked");
	}
});
!
999px

Console