Merge _s

  _s: Simplify caption styles to work in HTML5 mode.
  _s: Add context for translations and styles.
  Add tab to indent 404.php(L43 to L45).
  Remove style redundancy for menu toggle; cursor pointer is already defined for buttons.
  a11y improvements on primary navigation menu toggle
  Add support for html5 caption
  Update _s.pot : add "Primary Menu" in header.php(L31).