Created Issue: Visual editor doesn't support tag [2046]

Source: https://compositec1.codeplex.com/discussions/570659#post1316702
Registered in HQ TFS as Issue 4768

Steps to reproduce the issue:

1. Edit a page.
2. Switch to Source mode
3. Wrap the content with <main/>
4. Switch back to Visual mode

Expected: You can switch back to Visual mode. All the content is in place
Actual: An error message pops up: "Error in source code: Unknown error."

Markup example:

```
<main>
<h1>About us</h1>
<p>Here you can introduce your company.</p>
</main>
```