mastodon/app/views/layouts/application.html.haml

!!! 5
%html{:lang => 'en'}
  %head
    %meta{:charset => 'utf-8'}/
    %meta{:name => 'viewport', :content => 'width=device-width, initial-scale=1'}/
    %meta{'http-equiv' => 'X-UA-Compatible', :content => 'IE=edge'}/

    %title
      = "#{yield(:page_title)} - " if content_for?(:page_title)
      Mastodon

    = stylesheet_link_tag    'application', media: 'all'
    = csrf_meta_tags

    = yield :header_tags

  %body{ class: @body_classes }
    = content_for?(:content) ? yield(:content) : yield