Add this inside config.rb

Add custom classes to Middleman pages

def page_classes
  path = request.path_info.dup
  path << settings.index_file if path.match(%r{/$})
  path = path.gsub(%r{^/}, '')

  classes = []
  parts = path.split('.')[0].split('/')

  parts.join(' ')
end

Call it in the template using HAML

%section{class: "#{page_classes}"}