On the introduction to CSS there is a warning note about possible lessons failing.
Note: Some browser extensions may interfere with elements on the page. If the tests fail, try disabling your extensions for the most reliable experience.
What do they want disabling? what extensions?
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Extensions that change the appearance of your page, like a dark mode extension, change the rendered CSS and break the test suite.
Looks like I be visiting here, screaming for help on this one!
We have no way of knowing what extensions someone might have installed, but we know that some extensions will modify the html on a page, which will cause the tests to fail. This note is there so that if a user is seeing tests fail, they know that they should try disabling any extensions that might modify pages that you view.