Oh, by "significant whitespace" I meant whitespace sensitive indentation, which I think Python and YAML are the only languages that has that feature.
The idea is fine, I guess, although I certainly don't care for it. Where it gets most nasty is that whitespace that looks the same (in your editor) might not be equal and will cause you pain.
This is why idiomatic Python only uses spaces for indentation.
First off, what editor could I end up using in 2026 that makes this a realistic problem? Even the most basic editors I know of have options to convert tabs to spaces automatically (and any responsible teacher will tell the student to indent with spaces), and to continue the previous line's indentation automatically.
Second, modern Python is stricter about this, and also gives clear error messages.