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.
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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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<button onclick="createClick()">Click to add a new element to the list </button><br>
<p class = "output">Output: </p>
const item = document.createElement("li");
item.innerText = "Item Three";
document.querySelector(".output").innerText += `\nA new element is appended to this list`;