Google announced that its AMP and mobile-friendly test tools both now support the ability to edit code and rerun the test, live. So if you want to make changes in its code editor to see how it will impact the AMP and/or mobile-friendly test, you can now do so.
Where are the tests? You can access each test online; the AMP test is over here and the mobile-friendly test is over here.
Why it matters. This new feature will allow webmasters and developers to quickly test changing a snippet of code in the testing tool, to see if it results in a positive outcome in the test results. You no longer need to make the change directly on your live web site and then run the test after your code is deployed to see if it will have a positive or negative impact on the test results.