This can't be done as the markup is not up to hallo.js or any other rich text editor. They just trigger inserting markup, but the inserting is done by the browser. You will get slightly different results from different browsers. A workaround is possible only by changing the markup at the end.
Edit for point to docs on comment question:
The problem is that this is behavior of browsers since IE invented design mode without being a standard. The whatwg wrote it in the standard for HTML5. To get more information on the topic itself read
http://www.quirksmode.org/dom/execCommand.html
http://dev.opera.com/articles/view/rich-html-editing-in-the-browser-part-1/
http://www.thismuchiknow.co.uk/?p=64
http://www.whatwg.org/specs/web-apps/2007-10-26/multipage/section-contenteditable.html
http://www.w3.org/TR/DOM-Level-2-Traversal-Range/ranges.html
http://www.w3.org/wiki/Web_Editing_APIs
http://annevankesteren.nl/2005/07/contenteditable
http://www.sitepoint.com/the-tragic-comedy-that-is-rich-text-editing-on-the-web/