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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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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 

              
                <!-- All modern browsers support the [constraint validation API](https://www.w3.org/TR/html5/sec-forms.html#the-constraint-validation-api), a series of JavaScript methods for validating form controls. -->
<div class="container">
  <div class="Header">
    <h2>Form Submission</h2>
    <h3>Introduction</h3>
    <p>This section is non-normative. When a form is submitted, the data in the form is converted into the structure
      specified by the <a
        href="https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#concept-fs-enctype">enctype</a>,
      and then sent to the destination specified by the <a
        href="https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#concept-fs-action">action</a>
      using the given <a
        href="https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#concept-fs-method">method.</a>
    </p>
  </div>
  <form action="/find.cgi" method=get>
    <input type="text" name="t">
    <input type="search" name="q">
    <input type="submit">
  </form>
</div>
<hr />
<!-- Form 2 -->
<div>
  <div class="container">
    <h2>Form 2 - Essay Submission</h2>
    <p>This attribute is useful to include "save" buttons on forms that have validation constraints, to allow users to
      save their progress even though they haven't fully entered the data in the form. The following example shows a
      simple form that has two required fields. There are three buttons: one to submit the form, which requires both
      fields to be filled in; one to save the form so that the user can come back and fill it in later; and one to
      cancel the form altogether.</p>
    <form action="editor.cgi" method="post">
      <p><label>Name: <input required name=fn></label></p>
      <p><label>Essay: <textarea required name=essay></textarea></label></p>
      <p><input type=submit name=submit value="Submit essay"></p>
      <p><input type=submit formnovalidate name=save value="Save essay"></p>
      <p><input type=submit formnovalidate name=cancel value="Cancel"></p>
    </form>
  </div>
</div>
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CSS 

              
                small success {
  visibility: none;
  border-color: green;
}

small error {
  border-color: red;
}
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JS 

              
                function check(input) {
  if (
    input.value == "good" ||
    input.value == "fine" ||
    input.value == "tired"
  ) {
    return input.setCustomValidity('"' + input.value + '" is not a feeling.');
  } else {
    // input is fine -- reset the error message
    return input.setCustomValidity("Success");
  }
}
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999px

Console