Donald Knuth: Using Indentation to Represent Program Structure

The objection that, in the midst of a long document, telling what level of indentation one is at would be difficult is easily addressed by having the editor show space marks or using 'fold lines' in print -- neither of which is difficult.

In practice, though, I never have any problem with indentation based syntax, because change in indentation is easy enough to detect, and that's good enough for anything I've ever had to read.

Indentation is merely horizontal positional information.

The idea of using position to convey information is very old - music is written using vertical position information.