!!! Strict is returning HTML 5 doctype.
I assume you're using Rails 3? If so, this isn't a bug. Rails 3 defaults to HTML5 output, and so Haml running under Rails 3 defaults to :format => :html5, which causes this.
I'm experiencing the same bug in sinatra
!!! Strict still outputs to the html doctype. actually changing the doctype to anything other than html 5 does not seem to modify it.
Do you have the :format option set to :html5? You can check by adding = haml_buffer.options[:format] to your template.