Why not stick to conventions and go with dashes? Underscores are out of place in Javascript and the DOM... If I'm going to be forced to use underscore_case in Rendr that's different than the rest of my codebase (or other codebases I don't control which are all camelCase in Javascript) it's going to feel more unclean from the start.

Or you could say I'm not forced to use it, but now I'm using camelCase for variables and underscore_case for data attributes... so the two don't match, so why have the non-standard underscore_case at all.

(This would be an example of one of the things that makes libraries feel cludgy, and one of the reasons I love how consistent Backbone is, like I was talking about further up the thread with Jashkenas.)