I've always been of the opinion that YAML is inappropriate as a configuration format, period. It has ambiguous parses, which is an immediate no-no.

I remember years ago I was writing my own parser for YAML and when I came across this problem I posted on a few mailing lists and got confirmation that the behavior is per-spec. I've never touched YAML since and still won't.

But I miss the days of XML, XSD, and dare I say it ... XSLT. XSLT is stupidly good at what it does as long as you don't abuse it.