HTML-based Indented Markup Language for Ruby

I've never experienced the HTML closing tag mismatch error over the past 10 years or so, thanks to Haml.However, the Haml (or Slim or whatever equivalent) syntax is very much different from that of HTML. And to be honest, I still rarely can complete my template markup works without consulting the Haml online reference manual.Oh, why do we have to learn another markup language syntax where we just want to generate HTML documents? HTML has tags. HTML has indentations. Why can't we just use them?.