This article was written in 2013. It might or it might not be outdated. And it could be that the layout breaks. If that’s the case please let me know.
Testing a website is easy. You save a file, reload a browser and see what happens. Testing an ePub is not so simple. But there are some tools and tricks that make it a bit faster. First of all, you will need Sigil (or something else if you know anything better). You can edit all the files within Sigil, which has a decent HTML and CSS editor. But you can also open the files in your editor of choice. This is probably a very good idea for most of us. The downside is that you will have to save your work twice: first you have to save the HTML or CSS file in your text editor, and then you have to save the ePub file in order to test it on different devices and readers. So far so good. Testing on devices is the real pain.
The fastest way to get the files into your device
I found out that the fastest way to get an ePub into an iPad or iPhone is by using Dropbox. (Many others knew this ages ago, I just started with epubs yesterday). Just make sure your ePub is in your dropbox folder. Every time you save it it will be updated on your devices as well. You can now click on an icon with a down-facing arrow and you will see all the applications that can read the file. This works with Readmill, iBooks, Stanza, Kobo and more. It’s not as fast as refreshing the browser, but it’s not as slow and frustrating as transferring the file thru iTunes. This works with .mobi files for Kindle as well. It does not seem to work with .azw3 files for Kindle.
I have only tested this on my iOS devices, but I assume this also works on Android and other operating systems.