In which file do I put the code?
I have tried in EditPage.php, in EditPageAction.php and in my index, but no change.
Where is this function called?

When I input content in editor in HTML-mode, then switch to Normal, and then back to HTML, I can see that the unwanted tags are added, even if I have not saved the page.
Can the editor be set to accept my code without altering it?