The web development is a very dynamically changing environment. Almost each month the browser vendors release updates and sometimes major, which often breaks existing properly working functionality and adds regressions in the controls.
In this fast changing world, we are always striving to provide the best suggestions, workarounds and fixes that are currently available.
For the concrete situation, there isn't much to do since this behavior is not only controlled by the underlying rich text editing engine of the browser, but if you leave some tags unclosed, this won't be a valid HTML/XHTML/HTML5 and we will receive complaints for that the editor does not correct the content, which is one of its top features - see the Built-in Content Filters article
I hope that I delivered clarification on this topic.
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