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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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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).

Add External Stylesheets/Pens

Any URLs 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 its URL and the proper URL extension.

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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.

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

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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.

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.

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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.

If you're using React / ReactDOM, make sure to turn on Babel for the JSX processing.

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HTML 

              
                <h1>Intro to front-end web development</h1>
<div class="columns">
  <article>
    <h2>What is HTML?</h2>
    <p>
      <a href="https://www.w3.org/TR/html5">HTML</a> is the language used to create the web pages you visit everyday. It provides a logical way to structure content for web pages.
    </p>
    <p>
      Let's analyze the acronym "HTML", as it contains a lot of useful information. HTML stands for HyperText Markup Language.
    </p>
    <p>
      A markup language is a computer language that defines the structure and presentation of raw text. Markup languages work by surrounding raw text with information the computer can interpret, "marking it up" for processing.
    </p>
    <p>
      HyperText is text displayed on a computer or device that provides access to other text through links, also known as "hyperlinks".
    </p>
  </article>
  <article>
    <h2>What is CSS?</h2>
    <p>
      CSS, or Cascading Style Sheets, is a language that web developers use to style the HTML content on a web page. If you're interested in modifying colors, font types, font sizes, shadows, images, element positioning, and more, CSS is the tool for the job!
    </p>
    <img src="https://c1.staticflickr.com/1/537/32398153402_2a7028b2bd_z.jpg" alt="A drawing of an empty browser window">
    <p>
      Although the <code>&lt;style&gt;</code> element allows you to write CSS code within HTML files, this mixture of HTML and CSS can result in code that is difficult to read and maintain.
    </p>
    <p>
      It's common for developers to add substantial amounts of custom CSS styling to a web page. When all of that CSS code is placed within a &lt;style&gt; element in an HTML file, you risk the following two things:
    </p>
    <ol>
      <li>Creating a large HTML file that is difficult to read and maintain (by you and other developers). Overall, this can result in an inefficient workflow.</li>
      <li>Maintaining a clear distinction between web page structure (HTML) and web page styling (CSS).</li>
    </ol>
    <p>
      Fortunately, the following solution will help you avoid creating large HTML files that mix in CSS code: a CSS file!
    </p>
    <p>
      HTML files are meant to contain only HTML code. Similarly, CSS files are meant to contain only CSS code. You can create a CSS file by using the .css file name extension, like so: style.css
    </p>
    <p>
      With a CSS file, you can write all the CSS code needed to style a page without having to sacrifice the readability and maintainability of your HTML file.
    </p>
  </article>
</div>
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