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

I might understand this objection for YAML, but for python, it's very clear: a block ends whenever the content on the next line is indented by four fewer spaces. (And lists and dicts are explicitly closed with brackets)

If you're talking about matching block starts to ends, then that's just as bad in json, if not worse; without an editor that does brace coloring, it's almost impossible to eyeball which brace goes to which block without indenting (which is the whole point of significant whitespace).

> Deeply nested XML, provided the elements don't have some 300 character set of attributes all jammed into the opening element, is vastly easier to read than YAML.

Not really. More often than not, tags are visual noise that make it harder to scan for the actual content.

I have to read a lot of deeply nested XML for part of my job, and I wrote a parser to display it as YAML precisely because it's more readable. If your data is so convoluted that you need xml tags to make sense of it, that's usually a sign you need to either refactor it, or provide a proper display tool.