Copy rich text from Firefox does not capture style

If I copy the rendering of a simple html table, with the style sheet in the head, from Firefox and paste it in say Thunderbird (in a message in html mode), the table is copied, but not the style, for example borders are not collapsed.

If I do the same from Chromium to Thunderbird, from the same html file, styling is copied as well.

Is this expected behaviour, is there some good reasons for this? Or a bug?
I can add the html file, screenshots etc… here, or create an issue on bugzilla.

I have created this issue with an example file and screen captures.

This is a longstanding deficiency. Unless the style rules themselves are included in the selection, the receiving program (assuming it understands HTML), only applies default styles. I think other browsers are injecting those external styles into inline style attributes on the elements when you copy them.

Correct, Chromium inject the style as demonstrated in my ticket.

I feel copy/paste of rich text is important and should “just work”. Is there anything that can be done to prioritize or help with this issue?