freeCodeCamp alternatives to codepen due to:

freeCodeCamp Support
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freeCodeCamp alternatives to codepen due to:“You need a valid referrer to Log In to CodePen”

This link explains clearly why this happens: https://blog.codepen.io/2017/10/05/regarding-referer-headers/

For the Responsive Web Design Projects we must use codepen.io, we get a message like this: “For now, the test suite only works in Chrome! Please read the README below in the JS Editor before beginning.” which I likely doubts is strictly related to this issue as I could have Chrome and yet still have a Refferer controler.

Are there any possibilities to have other playgrounds rather than codepen.io in the future?

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You use repl.it, codesandbox.io, jsbin.com. Also, you could host the project on your own website as long as the written source code is available somewhere (i.e. GitHub).