For non-existent templates :
- Show the following text under the template name using current styling: "This template doesn't
- Show "add undocumented parameter" input as is, but change the link to red, as the template's page doesn't exist. → Done via T272487.
For subst use case:
- Show the following text under the template name using current styling: "Template documentation and parameter names (if existing) are not available when editing templates using modifiers, such as subst."
Note: needs investigation into how this is detected.
For unrecognizable wikitext:
- Leave current state: when forbidden wikitext characters are used the input is red and the "Add template" button is disabled. Future work could consider adding a warning, but not included in this ticket.