Read https://tbranyen.com/post/clipboard-hijacking-with-html5 to learn all about it and this is just bad web code and nothing more.
--> Hope this clears that up for you (understanding is key here as there is no reason anyone will fix this anytime soon.)
I was on craigslist website and wanted to forward a job opening I saw to my husband. I typed in his email add to the 1st box (as sender) and copied the email using keyboard shortcuts (imac). When I did the keyboard function to paste the email add I thought I copied from the 1st box, a short phrase/message was pasted. It says "Let's chat for starters I'm jay matt 1199 at gee, wink wink". I opened Pages, opened a blank page and did the same keyboard shortcuts/command paste and the same phrase was pasted. I tried selecting a sentence from another website using the trackpad and used keyboard commands to paste what I copied to the same document on Pages, and the copying and pasting was successful from the other website.
My husband is denying that it was him who typed the chatting thingy sentence, that he never chatted with anyone. Is it possible that this is some sort of a virus, malware or something similar to that? I have never heard of any type of virus that works like this before. I would appreciate a response.