Make the Markdown styles consistent with the source text.
The Markdown styles no longer jump nearly as much when going from

editing to viewing. The text aligns nearly everywhere except when there

are non-visible Markdown formatting characters, like * or `.
The CSS we use for the diff viewer syntax highlighting is now being used

for source code in Markdown, which replaces prettify.css.
Also, in general, styles are just a bit cleaner and more tightened.
Due to some strange bugs in Firefox with its textarea, where things are

off by 1px in places, some Firefox-specific CSS rules had to be added to

compensate.

Tested with a variety of Markdown styles in Chrome and Firefox.