For brand new plugin activations, we should consider to default to the new Transitional mode. (As of v2.0, Transitional mode is the default when an AMP-compatible theme is active.) This would be instead of defaulting to the Reader mode, which is what the default would continue to be for existing users of the plugin when they upgrade. For new users, the Transitional mode can be much superior to the Reader mode because not only are same theme templates being used, but also any validation errors are actually reported to the user. In the Reader mode, validation errors are silently sanitized which is dangerous since it can mean ads and other critical site functionality is removed. (Not as of #4480.)

We should also consider defaulting to the new Transitional mode if the user has an active theme which is known to be compatible with AMP (e.g. the core themes). We can also consider the same for compatible. plugins. We could also consider doing this by default if no excessive_css validation errors are raised, as this should mean that the paired mode templates will at least be stylistically the same as the canonical templates.

This would hopefully increase the adoption of the new AMP theme support.

See also #1288.