A browser shows the finished page, not the markup that builds it. Copying from that page can bring links, inline styles, list structure and hidden elements along for the ride.
It is not a broken document
The extra formatting is part of the source. It becomes a problem when the destination only needs the words, such as a manuscript, spreadsheet cell or plain-text email.
Make the destination simpler
Paste the source into whyaiai and choose the structure you want to keep. The converter removes the markup without changing the visible wording.
Remove hidden HTML formatting before you paste again.