I have the same thing happening. But: Neither have I Avast installed on this PC nor do I use Safari. It's Chrome. And no Avast plugin installed.
How is this possible?
I observed it on a Joomla site with JCE as editor (JCE works on top of TinyMCE).
EDIT: Happens on Firefox, too.
EDIT2: I investigated further and found that it is the HTML validator in JCE that inserts that code.