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
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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
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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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<title>Le doctype HTML en 1ère ligne</title>
<p>L'instruction doctype HTML est la première ligne de code du document.</p>
<p>Comme les informations renseignées dans l'élément head, "doctype HTML" est invisible à l'écran quand le navigateur ouvre la page web.</p>
<p>Grâce à la mention doctype HTML, ici, le navigateur sait qu'il doit utiliser le langage HTML5 pour interpréter le code.</p>
<p>Il n'y a pas besoin d'écrire de balise fermante pour l'instruction doctype HTML.</p>