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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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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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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.

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HTML 

              
                <div class="container container--spaced">
  <label>Using relative Pixel values</label>
  <div class="test">
    <div class="imagebox">image</div>
    <h1>Lorem ipsum dolor sit amet, consectetur adipisici elit.</h1>

    <p>Lorem ipsum dolor sit amet, consectetur adipisici elit, sed eiusmod tempor incidunt ut labore et dolore magna
      aliqua. Nec dubitamus multa iter quae et nos invenerat. Idque Caesaris facere voluntate liceret: sese habere.
      Inmensae subtilitatis, obscuris et malesuada fames. Morbi odio eros, volutpat ut pharetra vitae, lobortis sed
      nibh.</p>
  </div>
</div>

<div class="container container--centered">
  <span class="help">Change your browser's default font size in the settings to see a difference here.</span>
</div>
<hr>

<div class="container container--spaced">
  <label>Using absolute Pixel values</label>
  <div class="test use-px">
    <div class="imagebox">image</div>
    <h1>Lorem ipsum dolor sit amet, consectetur adipisici elit.</h1>

    <p>Lorem ipsum dolor sit amet, consectetur adipisici elit, sed eiusmod tempor incidunt ut labore et dolore magna
      aliqua. Nec dubitamus multa iter quae et nos invenerat. Idque Caesaris facere voluntate liceret: sese habere.
      Inmensae subtilitatis, obscuris et malesuada fames. Morbi odio eros, volutpat ut pharetra vitae, lobortis sed
      nibh.</p>
  </div>
</div>
!

CSS 

              
                // PX to REM conversion function
@function rem($pixels) {
  // Divide the pixel value by 16 (default font size)
  // Mulitplication by 1rem is needed to set the correct unit for the resulting value
  @return ($pixels / 16) * 1rem;
}

// HTML and Body styling
html {
  // Set 16 px as the base font size, currently the default in all browsers
  // 100% = 1em = 1rem = 16px = 12pt
  font-size: 100%;
  font-family: sans-serif;
}

body {
  // Set the base font size of the body to 16px = 1rem
  font-size: 1rem;
}

// Typo styles
p {
  font-size: rem(18);

  .use-px & {
    // Use a fixed value of 18px for the element
    font-size: 18px;
  }
}

h1 {
  font-size: rem(35);
  margin: rem(10) 0 0;

  .use-px & {
    font-size: 35px;
    margin: 10px 0 0;
  }
}

// Dimension and spacing styles
.test {
  padding: rem(15);

  .use-px & {
    padding: 15px;
  }
}

.imagebox {
  width: rem(150);
  height: rem(150);
  margin-left: rem(50);
  margin-bottom: rem(20);

  .use-px & {
    width: 150px;
    height: 150px;
    margin-left: 50px;
    margin-bottom: 20px;
  }
}


//===========================================================================
// Styles without function for the font sizing, just to make things look nice
.container {
  position: relative;
  margin: 0 auto;
  padding: 0 rem(10);
  max-width: rem(1000);

  &--spaced {
    margin-top: rem(50);
    margin-bottom: rem(50);
  }

  &--centered {
    text-align: center;
  }
}

label {
  font-size: rem(9);
  text-transform: uppercase;
  font-weight: bold;
  color: #0078ff;
}

.test {
  background: #f5f5f5;

  &:after {
    content: "";
    clear: both;
    display: table;
  }
}

.imagebox {
  display: flex;
  justify-content: center;
  align-items: center;
  float: right;
  background: #ddd;
  color: #888;
}

hr {
  width: 100%;
  margin: 0;
  background: none;
  border: 0;
  border-bottom: 2px dashed #eee;
}

.help {
  display: block;
  transform: translateY(calc(100% - #{rem(3)}));
  font-size: rem(9);
  font-weight: bold;
  color: #999;
  line-height: 1;
}
!

JS
!
999px

Console