Currently, `HTMLEditor` joins inline elements if the tag name is same and the
specified CSS style is not different. However, this may cause loosing style
specified by `<font>` elements at joining 2 paragraphs etc. We should not
join `<font>` elements if they have:
* different values for `color`, `size` or `face`
* only one has `color`, `size` or `face`
I don't delete unnecessary testcases in `delete.js` and `forwarddelete.js` which
are same as adding tests because deleting them causes unexpected failures of
some following tests near the boundary of test chunks. Perhaps, we should
group them better in another bug.
Differential Revision: https://phabricator.services.mozilla.com/D167742
70bec1642d