Not validating the HTML may produce undesirable results. Here is an example:
- Add an HTML fragment to a page.
- Add some invalid content (e.g. <html></html>) and save.
- Check that the HTML tag is not removed, and a class is added to it (is this correct?).
- Edit that same fragment again, replacing the content added in 2 by <!-- (an open comment, with no closing counterpart) and save.
- Refresh the page.
- It is not possible to edit the fragment content anymore (edit controls don't show up).