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HTML

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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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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.

All packages are different, so refer to their docs for how they work.

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HTML 

              
                <head>
  <link rel="stylesheet" type="text/css" href=".css">
</head>
<body>
  <div id="header"><h1>This is a header</h1></div>
  <div id="nav">
    <ol>
      <h4>First Set of Navigation</h4>
      <li>This is the first item</li>
      <li>This is the second item</li>
      <li>This is the third item</li>
      <li>This is the fourth item</li>
    </ol>
    
    <ul>
      <h4>Second Set of Navigation</h4>
      <li>Random thing one</li>
      <li>Random thing two</li>
      <li>Random thing three</li>
      <li>Random thing four</li>
    <ul>
  </div>
  <div class="main"><p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Curabitur tristique turpis mollis nibh aliquam rhoncus. Praesent dui tellus, vehicula a nunc non, tempus gravida lorem. Lorem ipsum dolor sit amet, consectetur adipiscing elit. In dapibus pretium ipsum, dictum tempus dolor fringilla ut. Aliquam pharetra rhoncus accumsan. Integer mollis pretium euismod. Nunc bibendum massa turpis, in sodales est ornare sit amet. Maecenas leo risus, ultricies id velit eget, gravida feugiat est. Mauris id elementum metus. Sed vel nibh in ex commodo rhoncus eu eu nunc. Mauris interdum tincidunt sem, a tincidunt felis scelerisque pellentesque. Etiam pulvinar lacinia ipsum, sed pellentesque orci laoreet ut. Aenean pellentesque sem nec orci euismod, et varius urna ultricies. Nullam maximus, ipsum sit amet consequat rhoncus, est quam placerat velit, a tempus lectus nibh eu est. Cras consectetur imperdiet magna, non ultricies leo sollicitudin sit amet. Suspendisse ullamcorper metus vitae porta rhoncus.</p></div>
      <div id="footer"><p>just some text</p></div>
<body>
!

CSS 

              
                #header {
  display:block;
  background-color:blue;
  height:10%;
  width:100%;
  font-family:Arial;
  border:2px solid black;
  border-radius:15px;
  text-align:center;
}

#nav {
  height:100%;
  background-color:purple;
  width:30%;
  float:left;
  border:2px solid black;
  border-radius:15px;
}

.main {
  float:right;
  width:69%;
  height:80%;
  position:relative;
}

#footer {
  display:block;
  width:100%;
  height:10%;
  background:yellow;
  border:2px solid black;
  position:relative;
  clear:both;
  border-radius:15px;
  padding: 10px;
}
!

JS
!
999px

Console