Replace consecutive empty lines with a single empty line. When writing to a terminal, act as if consecutive empty lines were a single empty line. The description of where the displayed line goes is wrong, historically. If the tags file contained a pattern, the first line of the first screen displayed shall be the line containing the first occurrence of the pattern in the file. All historical implementations of more handled nul bytes, and it's not acceptable that future implementations be limited.
Error messages are historically in the same place as the prompt.
There's no specification of what happens if an error message or user input line is longer than a single logical line on the screen. This is clearly a problem, as most terminals will scroll the screen. My guess is that this should be left up to the implementation, but there needs to be some wording of some kind. Historically, the mark command also had a user input prompt. The -c option did not historically affect the normal scrolling of more, i.e. the 'j' command didn't cause the screen to redraw.