Validation while using custom engine

[punqbund]

I am using MJML App 2.10.0 based on MJML v^4.1.2 on macOS. When I switch to a custom engine v.4.3.1 installed via npm (/Users/punqbund/node_modules/.bin/mjml), the code validation stops working.

Example: no error messages when trying to use mj-section inside of the other mj-section.

I would like to see the validation working with custom engines.

[kmcb777]

Actually when using a custom engine the app will process the mjml using the cli output, so it could be possible but requires to exec the cli a second time with the --validate option and parsing the output.
Maybe we should add a separate option to activate this since it will make the renders slower