Haml is literally a one-to-one relationship with HTML tags, so there's almost zero mental overhead. If anything, the tight relationship between indentation of HAML and outputted HTML makes it far easier to debug things, and I haven't had to debug a pesky missing closing tag bug in eons.<p>Sass is slightly more difficult, but by default in development mode it outputs comments describing which Sass rules generated the CSS you're looking at, so it's not too difficult to go back - and it's a far simpler language than whatever you're coding your app in.<p>Honestly, I don't see it as being any different than (as you point out) translating the outputted HTML/CSS back into ERB/PHP/etc, or as difficult as trying to trace output backwards through your codepath.