SGML actually has some really nice properties explicitly designed to better handle parsing invalid files. The redundant close tags, for example, allow <b>this is bold <i>and italic</b> but not bold</i> -- these sorts of errors are made by people and it was a goal to handle those sorts of situations. Whether or not that was a good goal? Well, I guess that's part of this debate.

So, how do you parse invalid YAML unambiguously?

Don't insult SGML!! SGML is actually quite nice. With omitable and case-insensitive tags, its pretty easy to work with if it is used in its intended domain: document markup.

YAML is great for exchanging data but SGML and YAML target different problems.