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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

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

About the <head>

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. If you need things in the <head> of the document, put that code here.

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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.

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

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HTML 

              
                <header><nav>Nav</nav></header>
<main>
  <h1>Header</h1>
  <p>This is paragraph 1</p>
  <p>This is paragraph 2</p>
  <p>This is paragraph 3</p>
  <p>This is paragraph 4</p>
  <p>This is paragraph 5</p>
  <p>This is paragraph 6</p>
  <p>This is paragraph 7</p>
  <p>This is paragraph 8</p>
  <p>This is paragraph 9</p>
  <p>This is paragraph 10</p>
  <p>This is paragraph 11</p>
  <p>This is paragraph 12</p>
  <p>This is paragraph 13</p>
  <p>This is paragraph 14</p>
  <p>This is paragraph 15</p>
  <p>This is paragraph 16</p>
  <p>This is paragraph 17</p>
  <p>This is paragraph 18</p>
  <p>This is paragraph 19</p>
  <p>This is paragraph 20</p>
  <p>This is paragraph 21</p>
  <p>This is paragraph 22</p>
  <p>This is paragraph 23</p>
  <p>This is paragraph 24</p>
  <p>This is paragraph 25</p>
  <p>This is paragraph 26</p>
  <p>This is paragraph 27</p>
  <p>This is paragraph 28</p>
  <p>This is paragraph 29</p>
  <p>This is paragraph 30</p>
  <p>This is paragraph 31</p>
  <p>This is paragraph 32</p>
  <p>This is paragraph 33</p>
  <p>This is paragraph 34</p>
  <p>This is paragraph 35</p>
  <p>This is paragraph 36</p>
  <p>This is paragraph 37</p>
  <p>This is paragraph 38</p>
  <p>This is paragraph 39</p>
  <p>This is paragraph 40</p>
  <p>This is paragraph 41</p>
  <p>This is paragraph 42</p>
  <p>This is paragraph 43</p>
  <p>This is paragraph 44</p>
  <p>This is paragraph 45</p>
  <p>This is paragraph 46</p>
  <p>This is paragraph 47</p>
  <p>This is paragraph 48</p>
  <p>This is paragraph 49</p>
  <p>This is paragraph 50</p>
  <p>This is paragraph 51</p>
  <p>This is paragraph 52</p>
  <p>This is paragraph 53</p>
  <p>This is paragraph 54</p>
  <p>This is paragraph 55</p>
  <p>This is paragraph 56</p>
  <p>This is paragraph 57</p>
  <p>This is paragraph 58</p>
  <p>This is paragraph 59</p>
  <p>This is paragraph 60</p>
  <p>This is paragraph 61</p>
  <p>This is paragraph 62</p>
  <p>This is paragraph 63</p>
  <p>This is paragraph 64</p>
  <p>This is paragraph 65</p>
  <p>This is paragraph 66</p>
  <p>This is paragraph 67</p>
  <p>This is paragraph 68</p>
  <p>This is paragraph 69</p>
  <p>This is paragraph 70</p>
  <p>This is paragraph 71</p>
  <p>This is paragraph 72</p>
  <p>This is paragraph 73</p>
  <p>This is paragraph 74</p>
  <p>This is paragraph 75</p>
  <p>This is paragraph 76</p>
  <p>This is paragraph 77</p>
  <p>This is paragraph 78</p>
  <p>This is paragraph 79</p>
  <p>This is paragraph 80</p>
  <p>This is paragraph 81</p>
  <p>This is paragraph 82</p>
  <p>This is paragraph 83</p>
  <p>This is paragraph 84</p>
  <p>This is paragraph 85</p>
  <p>This is paragraph 86</p>
  <p>This is paragraph 87</p>
  <p>This is paragraph 88</p>
  <p>This is paragraph 89</p>
  <p>This is paragraph 90</p>
  <p>This is paragraph 91</p>
  <p>This is paragraph 92</p>
  <p>This is paragraph 93</p>
  <p>This is paragraph 94</p>
  <p>This is paragraph 95</p>
  <p>This is paragraph 96</p>
  <p>This is paragraph 97</p>
  <p>This is paragraph 98</p>
  <p>This is paragraph 99</p>
  <p>This is paragraph 100</p>
</main>
!

CSS 

              
                :root {
  --nav-font-size: 100px;
}

body {
  position: relative;
  margin: 0; 
  font-family: sans-serif;
  text-align: center;
  /* Uncomment below if using position fixed */
  /*padding-top: var(--nav-font-size);*/
}

header {
  position: sticky;
  /*position: fixed;*/
  top: 0;
  width: 50%;
  background-color: rebeccapurple;
  font-size: var(--nav-font-size);
}
!

JS
!
999px

Console