Aren't there places where they mean different things? i.e. None means "use the rcParams default and "none" means "use an appropriate null value"?

This is the case at very least for facecolor, probably others...

This is about capitalization of strings 'None' vs. 'none' (I'll add better formatting above). You are right that None is something completely different.

Oops! Missed that entirely, sorry for noise.

I have definitely encountered this in several places (including markers). For now I've been erring on the side of accepting both "None" and "none" as synonyms if they already were used before, but only documenting that we accept 'none'. Sounds like this is completely in line with your proposal.

I'd add my vote for 'none' to also be added in places where it is not currently accepted, and documentation updated.

I was playing around with this and noted two things:

1. line styles are converted into "None" as the preferred representation:
   

   matplotlib/lib/matplotlib/lines.py

   Lines 1112 to 1113 in 3699ff3

   	if ls in [' ', '', 'none']:
   	ls = 'None'

2. Some code relies on that:
   

   matplotlib/lib/matplotlib/axes/_base.py

   Lines 474 to 477 in 3699ff3

   	# Note: We don't val.lower() != 'none' because val is not
   	# necessarily a string (can be a tuple for colors). This
   	# is safe, because *val* comes from _process_plot_format()
   	# which only returns 'None'.

Yes it’s a bit of a mess.