The Atto editor does not create accessible, or even consistent, HTML5 content code in all web browsers. When used in Chrome, the dominant web browser in the world these days, the HTML code it produces is seriously non-compliant with WCAG 2.0/2.1 level AA.

Here are just a few examples:

The <i>, and <b> should not be used, <em> and <strong> are preferred.
http://www.w3schools.com/tags/tag_b.asp

The <strike> tag is not HTML compliant.
http://www.w3schools.com/tags/tag_strike.asp

The <u> is not recommended.
http://www.w3schools.com/tags/tag_u.asp

Paragraphs should be defined with a <p> rather than a <div> with <br />. This is important for styling purposes.

The <li> tags should not end with a <br></li>, just </li>.

All web browsers should generate the same code. Otherwise, pick a cross-platform browser like Chrome or Firefox (even Edge is now based on Chromium with Safari to follow suit) and openly tell people know what browser they will need to use in order to generate accessible HTML in Atto.