You could've figured it out because the Stack Exchange network is the only site affected. Go to any other site, and the duck is gone.
This tells you the problem is on Stack Exchange, not on your computer, which excludes malware being the problem.
Use any CSS customization browser extension and change the style of .tagged-interesting, e.g.
background-color: red !important;
Make sure you declare your changes !important so they don't get overridden by the site style.
To customize the presentation of ignored tags, change the style of .tagged-ignored.
The webextension tag would only be useful when used in conjunction with other more specific tags such as an actual product tag, as such it would be a meta tag.
Problems with programming issues for the webextension API interface would off-topic here and be better off on Stack Overflow.
I don't see a compelling reason to create this tag at the moment.