Indentation by tab is good for shallow data or code e.g. Python. Space is bad. For deeper levels, I'd go for a visible, countable symbol Level 1 .Level 2 ..Level 3

This format seems to support one-space indents, so you could use an editor which highlights spaces easily and set indent level to 1 for visibility. The alternative is unfriendly config or another method of doing the same thing, and both are antithetical to having a simple data format.