I think it would be a good idea when you play online-standard, as moves start the opening ECO got displayed in a window. That would ease users work with opening books and database.

That is a good idea diogens!

Brilliant

or as Chess.com people will say

" Inconcievable"

It should be not difficult to implement, the data is already collected as it appears when you download a PGN.

It could show out in the moves section, just beneath the FEN window. That would help also lower rated players to be aware of which opening they are playing and perhaps push their opening education and curiosity.