That's only true for minimally formatted documents. For anything that approaches professional typesetting requirements, XML is a nightmare.

By far the biggest problem, it the requirement that inner elements must be closed before outer ones can be. This frequently means that the software must do a huge amount of read-ahead to figure out which aspect of the formatting changes first to make that formatting element innermost.

Sometimes, that's simply not possible to arrange and so you have to close a whole bunch of elements and then reopen all but one of them.

All this because a constraint of the format.

Ideal formats, such as used by typesetting systems that don't use XML, allow you to say: keep this formatting trait on until it's switched off. There is no concept of every element needing to be a subset of its encompassing element.