I can confirm that the HTML editor looks blank on Chrome, both on OSX and Windows 7. Works fine in Firefox, so I suggest that as a temporary solution.
I also had this issue, as did everyone on my team - any HTML editor would appear blank. We were all using Chrome and logged a Marketo support ticket about it.
They told us to clear our browser caches and that did the trick for all 3 of us.
Thanks Jace! To be honest, clearing the cache did not work for me initially. But for whatever reason, the HTML editor is now back on-line in Chrome. I'll mark Kassia's comment too, though.