In #20974 a new data-ampdevmode attribute was added which tells the validator to suppress reporting validation errors for elements it appears on, except for the root html element:

In such case, however, when there is only one validation error and it is due to being in debug mode, it would be very useful if the Validator extension would not show an error icon but rather a debug icon, similar to how it shows a warning icon when there are deprecation issues.

If there are other validation errors other than the one data-ampdevmode on the root html element, then the Validator extension should continue to show the error icon.

By showing a debug icon, users won't get alarmed when the system is generating markup that is explicitly invalid for dev mode purposes.

Originally suggested in #20974 (comment).