Hi. It's the way how TinyMCE cleans the text up. I don't know whether it will help but I would try to input these divs as:

<div>
  <br/>
  <div>
      Lorem ipsum
  </div>
  <br/>
</div>

You can specify what content and sub-content are allowed in the editor by modifying the file "tinyMceConfig.config".

If you install the package "Config Tree" by Tim Geyssens, you can find all the config files under the "developer" section.

 Also, try to visit this post on how-to customize the editor

Actually, I've realised that it seems to be a feature of the TidyNet library and unfortunately it cannot be diverged. In fact the TidyNet itself is rather outdated itself and also used inside umbraco in completely uncustomisable way. The only thing you can do is to turn it off at all in umbracoSettings.config with setting TidyEditorContent to false there, but I'm not sure if it's a good idea.

You also mention that you are new to Umbraco so you might also like to consider structuring your solution slightly differently. I try and limit what my editors need to understand and format in the RTE to as basic as possible - usually paragraphs and UL/OL lists is sufficient.

In this case I would move the nested DIVs to the master template (or XSLT/razor macro).

Cheers

Paul

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