What steps are needed to reproduce the bug?

1. Open Firefox
2. Disable third-party (cross-site) cookies via Firefox's main settings UI
3. Go to https://stylelint.io/demo/
4. Enter any CSS into the left pane
5. Notice that the right pane does not reflect the code in the left pane
6. Click on the Problems tab
7. Notice that the playground always reports No problems!

What did you expect to happen?

For the playground to work, even when third-party cookies are disabled.

What actually happened?

CSS code from left pane does not appear in right pane. CSS code errors are never found.