Seeking an elegant, CSS-only method for hiding/showing auto-height content (with transitions)

Try this, The anti-margin:

.wrapper_6 { min-height: 20px }
.wrapper_6 .activator {z-index:10; position: relative}
.wrapper_6 .content { margin-top: -100%; }
.wrapper_6 .activator:hover +.content{ margin-top: 0 }

http://jsfiddle.net/PWbXp/