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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About CSS Preprocessors
CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.
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.
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).
Add External Stylesheets/Pens
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<link rel="stylesheet" href="https://rocssti.net/downloads/rocssti-fr.css" integrity="sha384-IP5lCbdavBcGBez0nl3Vrt4DFlMhaL6n2fINeODGVVWdkI+qEJHUY5YFISHCFO+V" crossorigin="anonymous">
<p class="aligncenter">Coucou, SRI (Subresource Integrity est correctement appliqué), la CSS est chargée et la classe qui centre le texte marche !</p>