message: Fix margins around images and iframes
diff: iframe
  display block
  max-width 100%
  - margin 4em auto
  vertical-align bottom
  border 1px solid colors-gray-border
  + margin 2em auto
  iframe
  box-shadow 4px 4px 4px colors-gray-background
  + h2
  + margin-top 4em
  +
  hr
  border 1px solid colors-gray-border
  - margin 2.5em 2em
  + margin 3em 2em
  