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You have to be sure you included the codepen ressources (css, js scripts, etc. links) in your html.
In the CodePen you want to reproduce, check out the settings: the only resource used is the cdn link to the Vue dependency.
This kind of link targets a static file, here a javascript file, to download it manually instead of using some dependency manager like
npm
In the <head>
of your html, add this line:
<head>
<script src="https://unpkg.com/vue@next"></script>
</head>
Source:stackexchange.com