If you're a nitpicky reader like me then you probably know that not all current devices and apps support the current Epub standard. While the more popular devices like Nook, Kobo, and Sony can boast excellent support, there are many off-brand devices made by Chinese OEMs which support Epub to varying degrees, including at least one ereader maker (Gajah) which doesn't even support all of the CSS which can be used in an Epub ebook.
The varying and oftentimes unknown degree of Epub support is both frustrating for ebook creators and disappointing for readers, and that's why I am glad to see the new test suite. When it's done any reader should be able to download it, open it on an ereader or app, and then see all the specific ways where it comes up short.
This test suite grew out of the Epub3 archive which the IDPF launched a few months ago, and it is going to distill all of the individual ebooks, each of which demonstrates a single feature, into a smaller set of ebooks. I've never before posted a checklist of which features an app or ereader might be missing, but with the new test suite it should be easy to do.