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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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About the <head>

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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

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 

              
                <main>
  <header>Header</header>
  <nav>Nav</nav>
  <section>Section</section>
  <aside>Aside</aside>
  <footer>Footer</footer>
</main>
!

CSS 

              
                main {
  color: gainsboro;
  margin: 0 auto;
  max-width: 1000px;
  min-height: 100vh;
  display: grid;
  grid-template-columns: 100%;
  grid-template-rows: 100px
    50px
    1fr
    100px
    200px;
  grid-template-areas: 
    "my-header my-header my-header"
    "my-nav my-nav my-nav"
    "my-section my-section my-section"
    "my-aside my-aside my-aside"
    "my-footer my-footer my-footer"
}

header {
  grid-area: my-header;
  background-color: blueviolet;
}

nav {
  grid-area: my-nav;
  background-color: cornflowerblue;
}

aside {
  grid-area: my-aside;
  background-color: darkmagenta;
}

section {
  grid-area: my-section;
  background-color: indigo;
}

footer {
  grid-area: my-footer;
  background-color: darkorchid;
}

@media (min-width: 600px) and (max-width: 1000px) {
  main {
    grid-template-columns: 200px 1fr 200px;
    grid-template-rows: 150px
      100px
      1fr
      100px;
    grid-template-areas: 
      "my-header my-header my-header"
      "my-nav my-nav my-nav"
      "my-aside my-section my-section"
      "my-footer my-footer my-footer"
  }
}

@media (min-width: 1001px) {
  main {
    grid-template-columns: 200px 1fr 200px;
    grid-template-rows: 200px
      1fr
      100px;
    grid-template-areas:
      "my-header my-header my-header"
      "my-nav my-section my-aside"
      "my-footer my-footer my-footer"
  }
}
!

JS
!
999px

Console