Pen Settings



CSS Base

Vendor Prefixing

Add External Stylesheets/Pens

Any URL's added here will be added as <link>s in order, and before the CSS in the editor. You can use the CSS from another Pen by using it's URL and the proper URL extention.

+ add another resource


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.

+ add another resource


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.


Save Automatically?

If active, Pens will autosave every 30 seconds after being saved once.

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

Want to change your Syntax Highlighting theme, Fonts and more?

Visit your global Editor Settings.


  Browsers totally ignore 
  CSS property/value declarations 
  that they don't understand –
  falling back to earlier declarations.
  But sometimes variables <em>look valid</em>
  at parse-time,
  and are later discovered to be 
  <a href="">invalid at computed value time</a>.

  <strong>Older browsers</strong>
  (without CSS Variable support)
  will always ignore declarations
  that contain variables.
  <strong>Modern browsers</strong>
  (with CSS Variable support)
  will ignore if the <em>syntax</em> is clearly invalid
  at parse-time.
      If the value is <em>missing</em>
      (or set to <code>initial</code>),
      it will try the provided fallback.
      [<strong>Type Error</strong>]
      If the final value 
      doesn't match the property,
      it becomes <code>unset</code> —
      falling back to the 
      inherited or default value.



                html {
  color: red;
  /* Invalid Syntax is ignored */
  /* - all browsers: red */
  color: not a valid color; 
  color: var(not a valid variable name); 
  /* Wrong "Type" Values == `unset` */
  /* - modern browsers: black (unset) */
  --invalid-value: not a valid color value;
  color: var(--invalid-value, rebeccapurple);

  /* Initial Values == fallback */
  /* - modern browsers: rebeccapurple (fallback) */
  --undefined-variable: initial;
  color: var(--undefined-variable, rebeccapurple);

/* browser support flag */
html::before {
  content: '⚠ This browser DOES NOT support CSS Variables…';
  background: maroon;
  color: white;
  display: block;
  padding: 1em;

@supports (--css: var) {
  html::before {
    content: '✔ This browser DOES support CSS Variables…';
    background: darkgreen;

html {
  font-size: calc(1em + 1vw);

body {
  padding: 1em;