Fix messages with "html { height: 100%; }" set getting 1px-high views.
Fix messages with "html { height: 100%; }" set getting 1px-high views.

Partially reverts commit d4e2b84.

* ui/conversation-web-view.css: Reinstate triggering CSS 2.1 §
  10.6.7. for the HTML element.

tshikaboom/geary,tshikaboom/geary,tshikaboom/geary