the problem with html/template is that it's made for Go programmers. A good template language is one that's made for front end devs and designers. html/template tends to be painful in practice because it's too difficult for a non-gopher to use.
I understand the point you're making - I just want to point out that Haml is explicitly designed for HTML, whereas html/template more or less "just happens" to be useful for HTML.
As for ERB, I'm not convinced that it is too different in this regard. I'm not much of a Rubyist, and I find it incredibly frustrating to use, but perhaps that's a matter of personal preference.
{text,html}/template have their warts - I literally started Jetpack because of an unusual html/template quirk I ran into that was tricky to debug. However, I don't think it's any worse than other template language in this regard (my main points of reference are Jinja2, ERB, Haml, and Scribble).