Atto produces invalid nested unordered (UL) lists

 Nested lists should be inside a <li> element of the list in which it is nested. At the moment, embedded <ul> tags appear at the same level as the <li> tags instead of inside.

HTML5 W3C ul spec: HTML5 ul. Note that a <ul> element may contain exactly zero or more <li> elements. 

Steps to reproduce:

1. Create a new page or label in Moodle
2. Click the Atto editor toolbar button to create a bulleted (unordered) list.
3. Type a few lines of content. A single word will do.
4. Press the Indent button in the Atto toolbar.
5. Type a few more lines of content.
6. Switch to the HTML view.

Instead of valid HTML similar to this:

<ul>

    <li>Item 1</li>

    <li>Item 2

        <ul>

            <li>Item 2.1</li>

        </ul>

    </li></ul>

Atto is producing this invalid HTML:

<ul>

    <li>Item 1</li>

    <li>Item 2</li>

    <ul>

       <li>Item 2.1</li>

    </ul>

</ul>

What is even worse is that, if you try to fix it, Atto just goes and re-arranges it all back to the way it was.

This creates an accessibility failure of WCAG 2.1, specifically criterion 4.1.1 Parsing.