I added a system message according to the template Grace Note provided, the source code of the message is the following
There will be a short network-wide outage starting on 8/6 19:00 UTC for <a href="http://blog.serverfault.com/post/stack-exchange-maintenance-this-saturday/">system maintenance</a>. <span style="color:red; font-weight:bold;">Do not panic.</span>
The red "Do not panic" is rendered red in the preview, but not on the actual live system message. It appears without any formatting. I could understand if the formatting for those message would be limited, but the preview should ideally reflect the restrictions that will be imposed on the live system message.