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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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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.

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HTML 

              
                <h1>Liquid layout</h1>
<article>
  <p>Instead of using fixed widths for your layouts you could make a flexible layout using percentages for your column widths. This will work in more situations than a fixed-width layout that only looks right at one specific size. These were called liquid layouts.</p>
  <p>But while a liquid layout will look good across a wide range of widths, it will begin to worsen at the extremes. On a wide screen the layout will look like it’s been stretched out too far. On a narrow screen the layout will look like it’s been squashed. Both scenarios feel uncomfortable.</p>
  <p>You can mitigate these problems by using <code>min-width</code> and <code>max-width</code> for your layout. But then at any sizes below the minimum width or above the maximum width, you’ve got the same issues you’d have with a fixed-width layout. On a wide screen there’d be unused space going to waste. On a narrow screen, the user would have to move the whole page left and right in order to see everything.</p> 
</article>
<aside>
  <p>The word “liquid” is just one of the terms used to describe this kind of layout. These kinds of designs were also called fluid layouts or flexible layouts. The terminology was as fluid as the technique.</p>
  <p>This example is using the CSS <code>float</code> property to create columns. That was a popular technique before CSS grid or flexbox existed.</p>
</aside>
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CSS 

              
                body {
  font-family: sans-serif;
  line-height: 1.5;
  padding: 0 16px;
}

article {
  width: 66%;
  float: left;
}

aside {
  width: 33%;
  float: right;
}

h1 {
  margin-bottom: 0;
}
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JS
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999px

Console