Basically, he's saying that tab's width should not be based on
counting character, which is a low level concept. Instead, hitting Tab
should create a table cell of a kind, which spans multiple lines. Then
the editor will always keep aligned any text on adjacent lines which
is entered after the nth tab, no matter how long the "cell"'s text
length is and even if the font is not monospaced. Think about what
align-regexp does, but done live and working with proportional fonts
as well. Indeed, the demo Java applet uses a proportional font, and if
you move any end of line comment belonging to a block of lines, all
other comments move to keep perfectly aligned.

I think that is quite tricky to implement, more so if you try to
retrofit an already existing mode. BTW I'm not asking for such an high-
tech solution. I'd be quite happy if indentation would work as I'm
entering lines when I'm using a proportional font, which I find easier
to read.