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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-.

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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Packages

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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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HTML 

              
                
<h1>What is a Pixel?</h1>
<p>
  Zoom the page in or out,
  to see how the
  <em>device pixel ratio</em>
  changes the relationship
  between these two boxes.
  On high pixel-density screens,
  there may already be several device pixels
  for every CSS pixel,
  before adjusting the zoom level.
  Currently on this page, 
  CSS pixels are <strong><output>1</output> times</strong>
  the size of device pixels.
</p>
<p>
  Zoom in or out to see that ratio change!
</p>

<h2>200 CSS Pixels:</h2>
<div class="css"></div>

<h2>200 Device Pixels:</h2>
<div class="device"></div>
!

CSS 

              
                div {
  --size: 200;
  aspect-ratio: 1;
}

.css {
  background: mediumvioletred;
  width: calc(var(--size, 100) * 1px);
}

.device { 
  background: teal;
  width: calc(var(--size, 100) * 1px / var(--dppx, 1)); 
}


@layer reset, base;

@layer base {
  body {    
    --text-small: clamp(0.938rem, 0.824rem + 0.568cqi, 1.25rem);
    --text-normal: clamp(1.25rem, 1.023rem + 1.136cqi, 1.875rem);
    --text-large: clamp(1.35rem, 0.818rem + 2.659cqi, 2.813rem);
    --text-xlarge: clamp(1.7rem, 0.784rem + 4.58cqi, 4.219rem);
    --gap: clamp(12px, 0rem + 3.75cqi, 24px);
    font-size: var(--text-normal);
    padding: var(--gap);
  }
}

@layer reset {
  * { box-sizing: border-box; }
  html { block-size: 100%; }
  
  body {
    margin: unset;
    min-block-size: 100%;
  }  
  
  img,
  svg {
    display: block;
    max-inline-size: 100%;
  }
  
  picture { display: contents; }
  
  input,
  button,
  textarea,
  select {
    font: inherit;
  }
}
!

JS 

              
                // adapted from MDN:
// https://developer.mozilla.org/en-US/docs/Web/API/Window/devicePixelRatio#monitoring_screen_resolution_or_zoom_level_changes

let remove = null;
const root = document.documentElement;
const output = document.querySelector("output");

const updatePixelRatio = () => {
  if (remove != null) {
    remove();
  }
  const mqString = `(resolution: ${window.devicePixelRatio}dppx)`;
  const media = matchMedia(mqString);
  media.addEventListener("change", updatePixelRatio);
  
  remove = () => {
    media.removeEventListener("change", updatePixelRatio);
  };

  output.value = window.devicePixelRatio;
  root.style.setProperty('--dppx', window.devicePixelRatio);
};

updatePixelRatio();
!
999px

Console