Rendering another MJML template in MJML template

[returnv01d]

Describe the bug
When MJML template contains in body another full MJML template (another <mjml>....</mjml>) then it does render only outer template (see screenshot) without throwing any errors.

To Reproduce
Steps to reproduce the behavior:

1. Create a file with this MJML code:

<mjml>
  <mj-body background-color="#d2f1f6">
    <mj-section>
      <mj-column>
        <mj-text color="#F45E43">Hello World</mj-text>

      </mj-column>
    </mj-section>
    
    <mjml>
      <mj-body background-color="#F45E43">
        <mj-section>
          <mj-column>
            <mj-text>Test</mj-text>
          </mj-column>
        </mj-section>
      </mj-body>
    </mjml>
  </mj-body>
</mjml>

2. Render it to HTML by doing 'mjml -r <path_to_file> --config.validationLevel strict`

Expected behavior
There should be at least error about illegal nesting or rendering both templates.

MJML environment (please complete the following information):

• OS: MacOS
• MJML Version 4.5.1
• MJML tool used - local npm MJML

Screenshots

Additional context
Embedding MJML in layout uncovered this issue.

[iRyusa]

Why would you expect this to work ?

[returnv01d]

I would expect it to throw an error, not to work

[iRyusa]

Ok then I misread, the issue first my bad.

Really weird that the validator doesn't catch this https://github.com/mjmlio/mjml/blob/master/packages/mjml-preset-core/src/dependencies.js#L8 this shouldn't be marked as a valid children 🤔

[returnv01d]

I made a fix for this issue: #2709