> There are plenty of times in Python and YAML I cannot visually figure out where some block ends.

To match the start of a Python block (with possibly other blocks nested within it) to its end: start at the (non-whitespace) column where the opening line begins, and go straight down until you find another line that starts in that column (or EOF).

Or you use an editor (or plugin) that just knows how to do this, just as you would with the balanced brackets. cf. https://vi.stackexchange.com/questions/7262 .

You can also just... not nest so deeply, or write so long. My functions rarely have more than ten lines are are usually not even inside classes; and I don't generally need more than three levels of indentation (that gets you, for example, a test within a loop within a function — which might well be better done with a comprehension anyway).

> XML prevents this by explictly stating what element was just closed.

Only for elements that cannot, even indirectly, contain other elements of the same element name. And for this limited safeguard, you double up every element name even in cases where it would otherwise be completely clear (which contributes to visual noise as the sibling comment points out). Of course, XML is designed for things that inherently need to be deeply nested. Your code usually doesn't.

https://en.wikipedia.org/wiki/Turing_(programming_language) tried the same idea. It didn't catch on.