This tool is NOT that much about YAML nor HTML. Those just happen to be the easiest targets I could think of for examining elegant methods for verification and manipulation of arbitrary data structures formed out of well defined parts. Over the last fifteen years I have often wanted to automate solutions to different problems with a compact set of such methods.
Forming HTML documents out of such data structures according to rules expressed in same way may be a good goal as I actually want to find out a combination of necessary methods not totally unlike but almost infinitely easier to use than, say, XSLT.