We can mark all emails using keyboard shortcut Ctrl + A in Yahoo! Mail. Are there any keyboard shortcut to unmark all emails?
Note: Keyboard shortcut Ctrl + A is not listed here.
Try pressing Up Arrow or Down Arrow.
Here is a list of the keyboard shortcuts available in Yahoo! Mail.
By unmark do you mean "Mark as Unread"?
Just press Ctrl+A again and that will deselect all messages.