Feature description

When Developer Tools are enabled, when a validation error occurs on the permalink for a post, a warning will be displayed in the editor:

Prior to 2.0, Developer Tools were not available for Reader mode. Since most sites are in Reader mode, many users are going to be seeing this warning notice for the first time. We need to explain to them why they are seeing this warning all of a sudden. They need to know that the validation errors were occurring before, but they just weren't being told about them.

I wrote a support topic about this to preempt support topics being opened, but we could do something even more preemptive inside of WordPress. We can link to the relevant documentation page on amp-wp.org. If one isn't yet created, we should do so.

Additionally, we may want to provide a link to turn off Developer Tools in the user profile. Something like, “Don't want to see these warnings? Turn off AMP Developer Tools in your user profile” (where the bolded text links to the checkbox).

This is closely related to #4668, where we can mention here the specific themes and plugins that are causing validation errors. If there are validation errors from the theme, the warning should probably advise switching to Reader mode. Validation errors from plugins could advise to evaluate whether the removed invalid markup is needed, whether the plugin should be suppressed (with a link to the Plugin Suppression section). There can be prompts to reach out to the respective support forums for the themes and plugins.

Also related to #3821, as this information should be displayed in the sidebar.

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry