Ok the title was full of acronyms, but if you're familiar with two of them, that'd be more than enough.
JSTL closed the abstraction gap between JSP and Java code by allowing developers to write Java logic in XML... Yuck!!!
HAML on the other hand accomplished the same goal for RHTML and Ruby code by allowing developers to write RHTML in Ruby... Yumm!!!
Ok, that is not entirely true, but it is close enough. HAML is a DSL not directly based on the Ruby language as it does rely on a compiler to process its syntax. Still, it is very DRY, tidy, and easy on the eyes compared to RHTML. Check out the HAML/RHTML showdown on HAML's home page for a good demo of that.
If you're already utilizing HAML on a project, please share your experience with us. I'd be curious to know if it's a TKO against the likes of XHTML, RHTML, and JSP.