I'm not bashing against Haml, all that I'm saying is that is not necessary. Lately Slim also has become quite popular, so that would involve also having the Slim version. And in the future it will be something different, but as long as HTML remains as the lingua franca of the web, then there's no really need.
I used Haml in the past, and I understand it's popular nowadays mostly because of Rails, but I hardly think it's widely-used. Just to name a few that use HTML or a template engine of their own: Blogspot, Tumblr and Wordpress, and these are much more widely-used than Haml.
It's not necessary to have the markup in a textarea on the demo page either when someone could just view the source and copy+paste it from there. But it's not about what's necessary, it's about what will be of utility to people.
Consider the audience of Bootstrap when you talk about usage numbers. I'd wager the number of people using Bootstrap + haml (either through Rails or standalone) in their apps outnumbers the ones using Bootstrap + Wordpress/Blogspot/Tumblr.
The site is perfect as it is, there's no need to add a unnecessary and non-standard format.