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HTML

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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

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About the <head>

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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-.

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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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HTML 

              
                <header class="container-fluid">
  I'm a header
</header>
<section class="container">
  <aside class="sidebar">
    <a class="btn btn-primary">toggle</a>
    <div class="menu">sidebar</div>
  </aside>
  <div class="col-md-9 col-md-push-3 content">
    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nullam consectetur, dui id faucibus suscipit, est dui porttitor arcu, id porttitor arcu nibh eget ligula. Nulla mollis magna tincidunt lobortis dapibus. Duis aliquet neque et finibus consequat. Vestibulum eget magna sapien. Duis magna ligula, rutrum nec neque facilisis, malesuada lacinia metus. Aenean a posuere lorem. Proin eget ornare leo. Nullam pulvinar tempor molestie. Mauris rutrum interdum consequat. Integer maximus, ex id sagittis porta, dui velit vestibulum ante, sit amet facilisis libero leo et urna. Curabitur ullamcorper ligula eu ipsum suscipit tristique. Aenean nec turpis non metus iaculis vehicula. Nulla varius commodo aliquam. Donec interdum lorem faucibus nulla finibus, eget consectetur ligula sagittis. Aliquam tincidunt mauris vel felis convallis lacinia. Phasellus euismod augue at justo bibendum suscipit.

Nullam cursus odio luctus, placerat lacus at, pharetra quam. Fusce posuere vel augue eget eleifend. Suspendisse potenti. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus nulla sapien, rhoncus in mollis in, lacinia non mi. Aliquam erat volutpat. Praesent sagittis auctor arcu vitae tempus. Nulla diam diam, euismod nec dolor a, suscipit luctus enim. Nunc lobortis tempor lacus, at elementum ligula pharetra iaculis. Donec accumsan consequat imperdiet. Nullam rhoncus nisi ex, vitae tincidunt quam congue euismod. Aliquam tristique tincidunt neque et gravida. Pellentesque pharetra leo tincidunt semper blandit. Aliquam erat volutpat. Nulla velit nulla, egestas id risus vel, faucibus molestie lectus. Fusce mattis vestibulum maximus.

In ultricies gravida ultrices. Duis rutrum finibus tincidunt. Donec varius neque eget sagittis semper. Praesent congue venenatis erat, non finibus augue sodales ut. Sed ac velit vehicula ante dapibus malesuada et ac sapien. Proin ornare eu felis nec sagittis. Quisque pulvinar odio sed quam elementum, scelerisque tristique magna ornare. Cras ac accumsan libero, ac sodales felis. Cras accumsan nibh ultricies, molestie lorem porta, gravida sem. Pellentesque accumsan ex mi, vel vulputate metus condimentum tempor. Integer vitae elit pretium, vulputate velit id, pellentesque metus. Orci varius natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Duis dignissim metus cursus sollicitudin efficitur. Vivamus eleifend diam quam, ut rhoncus dolor faucibus in.Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nullam consectetur, dui id faucibus suscipit, est dui porttitor arcu, id porttitor arcu nibh eget ligula. Nulla mollis magna tincidunt lobortis dapibus. Duis aliquet neque et finibus consequat. Vestibulum eget magna sapien. Duis magna ligula, rutrum nec neque facilisis, malesuada lacinia metus. Aenean a posuere lorem. Proin eget ornare leo. Nullam pulvinar tempor molestie. Mauris rutrum interdum consequat. Integer maximus, ex id sagittis porta, dui velit vestibulum ante, sit amet facilisis libero leo et urna. Curabitur ullamcorper ligula eu ipsum suscipit tristique. Aenean nec turpis non metus iaculis vehicula. Nulla varius commodo aliquam. Donec interdum lorem faucibus nulla finibus, eget consectetur ligula sagittis. Aliquam tincidunt mauris vel felis convallis lacinia. Phasellus euismod augue at justo bibendum suscipit.

Nullam cursus odio luctus, placerat lacus at, pharetra quam. Fusce posuere vel augue eget eleifend. Suspendisse potenti. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus nulla sapien, rhoncus in mollis in, lacinia non mi. Aliquam erat volutpat. Praesent sagittis auctor arcu vitae tempus. Nulla diam diam, euismod nec dolor a, suscipit luctus enim. Nunc lobortis tempor lacus, at elementum ligula pharetra iaculis. Donec accumsan consequat imperdiet. Nullam rhoncus nisi ex, vitae tincidunt quam congue euismod. Aliquam tristique tincidunt neque et gravida. Pellentesque pharetra leo tincidunt semper blandit. Aliquam erat volutpat. Nulla velit nulla, egestas id risus vel, faucibus molestie lectus. Fusce mattis vestibulum maximus.

In ultricies gravida ultrices. Duis rutrum finibus tincidunt. Donec varius neque eget sagittis semper. Praesent congue venenatis erat, non finibus augue sodales ut. Sed ac velit vehicula ante dapibus malesuada et ac sapien. Proin ornare eu felis nec sagittis. Quisque pulvinar odio sed quam elementum, scelerisque tristique magna ornare. Cras ac accumsan libero, ac sodales felis. Cras accumsan nibh ultricies, molestie lorem porta, gravida sem. Pellentesque accumsan ex mi, vel vulputate metus condimentum tempor. Integer vitae elit pretium, vulputate velit id, pellentesque metus. Orci varius natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Duis dignissim metus cursus sollicitudin efficitur. Vivamus eleifend diam quam, ut rhoncus dolor faucibus in.
  </div>
</section>
<footer class="container-fluid">
  I'm a footer
</footer>
!

CSS 

              
                html {
  height: 100%;
  box-sizing: border-box;
}

body {
  position: relative;
  margin: 0;
  padding-bottom: 6rem;
  min-height: 100%;
  *,
  *:before,
  *:after {
    box-sizing: inherit;
  }
}

header {
  background:red;
  position:fixed;
  width: 100%;
  z-index: 1;
}

footer {
  position: absolute;
  right: 0;
  bottom: 0;
  left: 0;
  background-color: blue;
}

aside {
  float:left;
  position: fixed;
  top: 40px;
  .menu {
    opacity:1;
    transition: all 0.2s ease-out;
    &.hide-me {
      opacity:0;
    }
  }
}

.content {
  margin-top: 40px;
}
!

JS 

              
                $(".btn").click(function(){
    $(".menu").toggleClass("hide-me");
});
!
999px

Console