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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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Using packages here is powered by Skypack, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ES6 import usage.

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HTML 

              
                <button id="ignore-my-existence">Give Me A Blue Box</button>

<!--

My own set of rules to make this more interesting (and challenging):

1) Each version must be the same size aligned to the middle.
2) No extra HTML. Do it with what every page has.
3) Only body element gets the box class, so you end up with only one element to play with.
 
-->
!

CSS 

              
                #ignore-my-existence {
  cursor: pointer;
  line-height: 1;
  position: fixed;
  right: 1em;
  top: 1em;
}

/* some base styles so there is something to work with */
html {
  background: white;
  color: white;
  height: 100%;
  width: 100%;
}

body {
  margin: 0;
}

body:before {
  content: 'Note: Currently only Firefox displays everything. WebKit\'s support for viewport units is bad. IE11 fails in flexbox and outline samples.';
  position: absolute;
  left: 1em;
  top: 1em;
  color: blue;
  text-shadow: 1em 1em 0 white;
  line-height: 1;
}

/* and so it begins... */

.box1:before { content: 'Relative (WebKit fails to calculate top: 20%;)'; }
.box1 {
  background: blue;
  margin: 0 20%;
  height: 60%;
  position: relative;
  top: 20%;
}

.box2:before { content: 'Absolute'; }
.box2 {
  background: blue;
  position: absolute;
  top: 20%;
  right: 20%;
  bottom: 20%;
  left: 20%;
}

.box3:before { content: 'Border + viewport units (WebKit fails: viewport units)'; }
.box3 {
  border-left: 60vw solid blue;
  height: 60vh;
  margin: 20vh 20vw;
}

.box4:before { content: 'Background-clip'; }
.box4 {
  background: blue;
  background-clip: content-box;
  height: 60%;
  padding: 20vh 20%;
}

.box5:before { content: 'Box-shadow (WebKit fails: viewport units)'; }
.box5 {
  background: blue;
  box-shadow: inset 20vw 20vh white,
    inset -20vw -20vh white;
  height: 100%;
}

.box6:before { content: 'Flexbox + pseudo child (fails on IE11)'; }
.box6 {
  align-items: center;
  display: flex;
  height: 100%;
  justify-content: center;
  width: 100%;
}
.box6:after {
  background: blue;
  content: '';
  height: 60%;
  width: 60%;
}

.box7:before { content: 'Inline-block pseudo child'; }
.box7 {
  line-height: 100vh;
  text-align: center;
}
.box7:after {
  background: blue;
  content: '';
  display: inline-block;
  height: 60vh;
  vertical-align: middle;
  width: 60vw;
}

.box8:before { content: 'Base64 image'; }
.box8 {
  background-image: url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAIAAACQd1PeAAAADElEQVQI12NgYPgPAAEDAQDZqt2zAAAAAElFTkSuQmCC');
  background-repeat: no-repeat;
  background-position: 20vw 20vh;
  background-size: 60vw 60vh;
  height: 100%;
}

.box9:before { content: 'Box-sizing + borders (WebKit fails: viewport units)'; }
.box9 {
  background: blue;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
  border-color: white;
  border-style: solid;
  border-width: 20vh 20vw;
  height: 100%;
}

.box10:before { content: 'BONUS! Outline (does not allow % or setting X and Y individually... and WebKit fails again, viewport units. IE11 fails too.)'; }
.box10 {
  background: blue;
  outline: 20vm solid white;
  outline: 20vmin solid white;
  outline-offset: -20vm;
  outline-offset: -20vmin;
  height: 100%;
}
!

JS 

              
                // http://www.a-blue-box.com/
// https://css-tricks.com/blue-box/

(function(newBox) {
  var index = 0,
      body = document.getElementsByTagName('body')[0];
  newBox.onclick = function() {
    index++;
    body.className = 'box' + index;
    newBox.innerHTML = 'Blue Box #' + index;
    index %= 10;
  };
})(document.getElementById('ignore-my-existence'));
!
999px

Console