Description of the need

When an advanced user changes an input format from Full HTML to any format with the Limit allowed HTML tags filter enabled, it is likely that something that was previously on the page will disappear. This can be alarming to people, especially when there was no warning that this would happen.

Proposed solution

When an advanced user changes an input format from Full HTML to any format with the Limit allowed HTML tags filter enabled, we should display a JS warning message to that user, letting them know that anything in that textarea that does meet the requirements of the new filter will be hidden from view.

I recommend something like The [Standard format] will not render HTML tags other than [list of allowed tags]. If any of those tags are included here, they will be saved but hidden from view

GitHub Link: 
https://github.com/backdrop/backdrop-issues/issues/4807
GitHub Issue #: 
4807