Hello,
in 4.1.8 and 4.3+ (and maybe in all 4.1 to 4.3 for now), links on images are improperly constructed in Atto editor used in Firefox browser (tested with latest ESR version).
The "a" tag is created, with its title attribute but no href attribute.
In Atto and a Blink based browser (like Brave or Chrome), theses links are correct : title and href attribute are present and filled with the URL provided by user. Same thing in TinyMCE text editor : Firefox and Blink's behave well with it.
NB. It's all about links on images. Links alone, as a text, works fine in Atto+Firefox.

To reproduce (with Firefox browser) :

1. In user preference, editor preferences, choose Atto HTML editor.
2. In a course, create or edit a "Text and media area" (ex-label).
3. In the area, use the tool bar to add an image.
4. Click on the image to select it and use the "Link" in the tool bar.
5. In the dialog box that opens, enter a URL and click "create link". It should fill the title automatically.
6. At bottom of edition page, validate with "Save and return to course".
7. Back in the course, use the browser inspector to check the link.

Results : the "a" tag only contains the title attribute (filled with the URL) but no href attribute. The link is not working.
Expected results : the "a" tag contains href and title attributes, filled with the URL.

NB. Same steps with Blink's browser (Brave, Chrome) or with TinyMCE gives the expected results.

Thanks by advance,
Regards,
Marina