Pen Settings
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.
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.
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.
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
Any URL's added here will be added as <link>s in order, and before the CSS in the editor. If you link to another Pen, it will include the CSS from that Pen. If the preprocessor matches, it will attempt to combine them before processing.
About External Resources
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.
If the stylesheet 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 CSS 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.
JavaScript
About JavaScript Preprocessors
JavaScript preprocessors can help make authoring JavaScript easier and more convenient.
Babel includes JSX processing.
Add External Scripts/Pens
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.
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.
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.
About Packages
Using packages here is powered by Skypack, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ES6 import usage.
All packages are different, so refer to their docs for how they work.
If you're using React / ReactDOM, make sure to turn on Babel for the JSX processing.
Details
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Auto-Updating Preview
If enabled, the preview panel updates automatically as you code. If disabled, use the "Run" button to update.
Format on Save
If enabled, your code will be formatted when you actively save your Pen. Note: your code becomes un-folded during formatting.
Editor Settings
Code Indentation
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Visit your global Editor Settings.
Template
Make Template?
Templates are Pens that can be used to start other Pens quickly from the create menu. The new Pen will copy all the code and settings from the template and make a new Pen (that is not a fork). You can view all of your templates, or learn more in the documentation.
Template URL
Any Pen can act as a template (even if you don't flip the toggle above) with a special URL you can use yourself or share with others. Here's this Pen's template URL:
Screenshot
Screenshot or Custom Thumbnail
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