If MediaWiki:default-manifest.json does not exist, then the manifest tag should not be added to the page. I think it's possible admins might install the extension but forget about it, so it would be a good idea to add a check in there for Title existence.
Also not sure what to do if the manifest is in an invalid format. If I edit with {"invalid": true} it is still registered as the manifest but it triggers site warnings. Not sure if that should be treated as GIGO.
Fixed! As to validating the manifest, that would be quite complicated and it's already a task most browsers will do themselves and give warnings about in the developer console.
If MediaWiki:default-manifest.json does not exist, then the manifest tag should not be added to the page. I think it's possible admins might install the extension but forget about it, so it would be a good idea to add a check in there for Title existence.
Also not sure what to do if the manifest is in an invalid format. If I edit with {"invalid": true} it is still registered as the manifest but it triggers site warnings. Not sure if that should be treated as GIGO.