Editor inserts class=”MsoNormal” into html tags

Searched and found a couple of instances in the forum where class=”MsoNormal”
is included in some example code, and each time the person was admonished not to include styles in their <p> tags.

Haven’t located a thread that mentions that it seems the WP editor is inserting these style classes. If Safari is used, a similar “apple normal” style is inserted into the code. Sometimes in <p>, other times in other html, such as list tags.

Does anyone have a clue about why this code is inserted and how to avoid it?

Thanks!