StrictYAML is great (and the author is in these comments!), but ultimately it's one specific library, not a format spec, so to depend on it for a project you need every person/tool doing the writing/parsing to commit to use that library (and the programming language it was written for).

Again, it's a great project, but I wanted something similar that is a language-agnostic format specification, so moved on to using NestedText wherever I can.