I have used the MAC editor Coda from Panic for a while as my preferred web site and template editor. I have recently been using it to edit ePUB innards.
Coda normally expects that you are editing a site on a server, or at least transferring onto a server after editing locally, so the configuration for a 'site' may not seem initially appropriate for editing ePUBs. Nevertheless, setting up a local folder is straightforward. In fact, you can set this up to be the root of the complete ePUB, thereby giving you the potential to edit the various XML files (package files and TOC files).
CSS3 provides some neat effects, like rounded corners and shadows, but InternetExplorer (before version 9) just ignores these CSS rules.
Is there a way to get Internet Explorer to conform?
Hopefully you are seeing the rounded corners and shadows, even if you are using Internet Explorer, because Jason Johnston has provided a solution and you can find out all about it on his web site: css3pie