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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.

!
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.

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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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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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Visit your global Editor Settings.

HTML 

              
                <nav class="toc">
	<ul>
		<li><a href="#topic1">Topic 1</a></li>
		<li>
			<a href="#topic2">Topic 2</a>
			<ul>
				<li><a href="#itemA">Item A</a></li>
				<li><a href="#itemB">Item B</a></li>
				<li><a href="#itemC">Item C</a></li>
				<li><a href="#itemD">Item D</a></li>
			</ul>
		</li>
		<li>
			<a href="#css">Topic 3</a>
      <ul>
        <li><a href="#itemE">Item E</a></li>
        <li><a href="#itemF">Item F</a></li>
        <li><a href="#itemG">Item G</a></li>
      </ul>
		</li>
	</ul>
  <svg class="toc-marker" width="200" height="200" xmlns="http://www.w3.org/2000/svg">
    <path stroke="#444" stroke-width="3" fill="transparent" stroke-dasharray="0, 0, 0, 1000" stroke-linecap="round" stroke-linejoin="round" transform="translate(-0.5, -0.5)" />
  </svg>
</nav>

<article class="contents">
  <section id="topic1">
    <h2>Topic 1</h2>
    <p>
      This page demonstrates an idea for how progress can be visualized inside of a standard page nav. Scroll the page and note how the marker animates to highlight all of the sections that are currently on screen.
    </p>
    <p>
      The rest of the content below is taken from <a href="https://slides.com/developers" target="_top">slides.com/developers</a>. Take a look at that to see how the progress nav looks on a real page.
    </p>
    <p>
      Created by Hakim El Hattab | <a href="http://hakim.se" target="_top">hakim.se</a> | <a href="https://twitter.com/hakimel" target="_top">@hakimel</a> 
    </p>
		<h2>Slides for Developers</h2>
		<p>
			We strive to make Slides a great and flexible tool for developers. Presentations created on Slides are HTML documents under the hood, so generally anything that HTML can do, Slides can do. We make it easy to access and edit the underlying HTML and CSS through the <a href="#dev">Developer Mode</a>.
		</p>
		<p>
			There's also an API for creating new presentations with preset content and we're aiming to add additional APIs in the future.
		</p>
	</section>

	<section>
		<div id="topic2">
			<h2>Topic 2</h2>
			<p>
				Next topic is discussed here!
			</p>
			<p>
				To enable the developer mode open the editor settings in the bottom left corner:
			</p>
      <img class="developer-image" src="https://static.slid.es.s3.amazonaws.com/site/developers/slides-developer-mode-1.png">
    </div>

		<div id="itemA">
			<h3>Item A</h3>
			<p>
				Here's where you get into more detail!
			</p>
		</div>

		<div id="itemB">
			<h3>Item B</h3>
			<p>
				Here's where you get into more detail!
			</p>
		<div id="itemC">
			<h3>Item C</h3>
			<p>
			Here's the text for another deep dive on topic 2.
			</p>
		<div id="itemD">
			<h3>Item D</h3>
			<p>
			Focus on clearly communication each point. 
			</p>
    <div id="topic3">
      <h2>Topic 4</h2>
      <p>
        Pay attention to the heading levels of topics vs. items
      </p>
      <p>
        The editor preprocesses styles using <a href="http://lesscss.org/">LESS</a>, though you're free to write plain CSS as well. We apply the styles in real-time as you type so there's no need to leave the editor or even press a refresh button to see what you're getting. Note that when your styles are saved they will be automatically wrapped in a ".reveal {}" selector to avoid conflicts with other page styles.
      </p>
    </div>
    <div id="css-fonts">
      <h3>Custom Fonts</h3>
      <p>
        You can load custom fonts from Typekit and Google fonts and apply them using custom CSS. <a href="http://help.slides.com/knowledgebase/articles/1077976-custom-fonts-pro">Find out more</a>.
      </p>
    </div>
    <div id="css-developer-mode">
      <h3>Developer Mode</h3>
      <p>
        If you turn on the <a href="#dev">developer mode</a> you can also add custom classes to any focused element. This is a convenient way to easily apply your CSS to specific elements.
      </p>
    </div>
  
    <div id="css-examples">
		<h3>Examples</h3>
		<pre class="code-block" data-highlight-theme="github-gist"><code class="hljs css">// Change presentation background
&amp; {
    background: #a83239;
}

// Change color of body text
.slides {
    color: #009999;
}

// Turn text white if the slide has a dark background
.has-dark-background {
    color: #fff;
}

// Add a blue border to all slides (each slide is a &lt;section&gt;)
.slides section {
    outline: 1px solid blue;
}

// Add a red border to all vertical stacks of slides
.slides section.stack {
    outline: 1px solid red;
}<br>
// Include a custom font<br>@font-face {<br>&nbsp; font-family: "Cabin Sketch";<br>&nbsp; src: url("https://s3.amazonaws.com/static.slid.es/fonts/cabinsketch/cabinsketch-regular.woff") format("woff"),<br>&nbsp; &nbsp; &nbsp; &nbsp;url("https://s3.amazonaws.com/static.slid.es/fonts/cabinsketch/cabinsketch-regular.ttf") format("truetype");<br>}
<br>.slides h1, .slides h2, .slides h3 {<br>&nbsp; &nbsp; font-family: "Cabin Sketch";<br>}


// Change color of controls and progress bar
@interactiveColor: rgb(200,105,119);

.progress span {
    background: @interactiveColor !important;
}

.controls .navigate-left {
    border-right-color: @interactiveColor !important;
}
.controls .navigate-right {
    border-left-color: @interactiveColor !important;
}
.controls .navigate-up {
    border-bottom-color: @interactiveColor !important;
}
.controls .navigate-down {
    border-top-color: @interactiveColor !important;
}</code></pre>
  </div>

	<p>Here's where you can access the CSS editor from inside of the Style panel:</p>
	<img class="developer-image" src="https://static.slid.es.s3.amazonaws.com/site/developers/slides-css-editor-1.png">

	<p>A screenshot of the editor:</p>
	<img class="developer-image" src="https://static.slid.es.s3.amazonaws.com/site/developers/slides-css-editor-2.png">

	</section>
</article>
!

CSS 

              
                $menuWidth: 14em;
$menuPaddingLeft: 3em;

* {
  box-sizing: border-box;
}

body {
  width: 100vw;
  height: 100vh;
  overflow: auto;
}

body {
  padding: 2em 2em 2em $menuWidth+$menuPaddingLeft;
  font-size: 16px;
  line-height: 1.6;
  
  font-family: 'Roboto', sans-serif;
}

.toc {
  position: fixed;
  left: $menuPaddingLeft;
  top: 5em;
  padding: 1em;
  width: $menuWidth;
  line-height: 2;
  
  ul {
    list-style: none;
    padding: 0;
    margin: 0;
  }
  
  ul ul {
    padding-left: 2em;
  }
  
  li a {
    display: inline-block;
    color: #aaa;
    text-decoration: none;
    transition: all 0.3s cubic-bezier(0.230, 1.000, 0.320, 1.000);
  }
  
  li.visible>a {
    color: #111;
    transform: translate( 5px );
  }
}

.toc-marker {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: -1;
  
  path {
    transition: all 0.3s ease;
  }
}

.contents {
  padding: 1em;
  max-width: 800px;
  font-size: 1.2em;
  font-family: 'Frank Ruhl Libre', sans-serif;

  img {
    max-width: 100%;
  }
  
  .code-block {
		white-space: pre;
		overflow: auto;
		max-width: 100%;

		code {
      display: block;
			background-color: #f9f9f9;
			padding: 10px;
		}
	}

	.code-inline {
		background-color: #f9f9f9;
		padding: 4px;
	}
  
  h2,
  h3 {
    padding-top: 1em;
  }
}


@media screen and (max-width: 1200px) {

  body {
    font-size: 14px;
  }
  
}
!

JS 

              
                var toc = document.querySelector( '.toc' );
var tocPath = document.querySelector( '.toc-marker path' );
var tocItems;

// Factor of screen size that the element must cross
// before it's considered visible
var TOP_MARGIN = 0.1,
    BOTTOM_MARGIN = 0.2;

var pathLength;

var lastPathStart,
		lastPathEnd;

window.addEventListener( 'resize', drawPath, false );
window.addEventListener( 'scroll', sync, false );

drawPath();

function drawPath() {
  
  tocItems = [].slice.call( toc.querySelectorAll( 'li' ) );

  // Cache element references and measurements
  tocItems = tocItems.map( function( item ) {
    var anchor = item.querySelector( 'a' );
    var target = document.getElementById( anchor.getAttribute( 'href' ).slice( 1 ) );

    return {
      listItem: item,
      anchor: anchor,
      target: target
    };
  } );

  // Remove missing targets
  tocItems = tocItems.filter( function( item ) {
    return !!item.target;
  } );

  var path = [];
  var pathIndent;

  tocItems.forEach( function( item, i ) {

    var x = item.anchor.offsetLeft - 5,
        y = item.anchor.offsetTop,
        height = item.anchor.offsetHeight;

    if( i === 0 ) {
      path.push( 'M', x, y, 'L', x, y + height );
      item.pathStart = 0;
    }
    else {
      // Draw an additional line when there's a change in
      // indent levels
      if( pathIndent !== x ) path.push( 'L', pathIndent, y );

      path.push( 'L', x, y );
      
      // Set the current path so that we can measure it
      tocPath.setAttribute( 'd', path.join( ' ' ) );
      item.pathStart = tocPath.getTotalLength() || 0;
      
      path.push( 'L', x, y + height );
    }
    
    pathIndent = x;
    
    tocPath.setAttribute( 'd', path.join( ' ' ) );
    item.pathEnd = tocPath.getTotalLength();

  } );
  
  pathLength = tocPath.getTotalLength();
  
  sync();
  
}

function sync() {
  
  var windowHeight = window.innerHeight;
  
  var pathStart = pathLength,
      pathEnd = 0;
  
  var visibleItems = 0;
  
  tocItems.forEach( function( item ) {

    var targetBounds = item.target.getBoundingClientRect();
    
    if( targetBounds.bottom > windowHeight * TOP_MARGIN && targetBounds.top < windowHeight * ( 1 - BOTTOM_MARGIN ) ) {
      pathStart = Math.min( item.pathStart, pathStart );
      pathEnd = Math.max( item.pathEnd, pathEnd );
      
      visibleItems += 1;
      
      item.listItem.classList.add( 'visible' );
    }
    else {
      item.listItem.classList.remove( 'visible' );
    }
    
  } );
  
  // Specify the visible path or hide the path altogether
  // if there are no visible items
  if( visibleItems > 0 && pathStart < pathEnd ) {
    if( pathStart !== lastPathStart || pathEnd !== lastPathEnd ) {
      tocPath.setAttribute( 'stroke-dashoffset', '1' );
      tocPath.setAttribute( 'stroke-dasharray', '1, '+ pathStart +', '+ ( pathEnd - pathStart ) +', ' + pathLength );
      tocPath.setAttribute( 'opacity', 1 );
    }
  }
  else {
    tocPath.setAttribute( 'opacity', 0 );
  }
  
  lastPathStart = pathStart;
  lastPathEnd = pathEnd;

}
!
999px

Console