I assumed that was the reason, it underscores the importance of the tool. It gives us an absolute measure - if it passes this test then the reader is at fault, and that helps for bug bashing and feature enhancement.
I have just come across Feedbooks I am very impressed by what you are doing there.
I have some suggestions for your epub software.
I suggest the mandatory use of rational paragraph numbering for IDs (ie Chapter I.35) referencing is important in some circumstances and page numbers are dead for this in ebooks.
Use the XMLnamespace effectively by using World Unique Ids (WUIDS) and not urls.
If you are interested in either of these drop me a line at firstname.lastname@example.org
PS WUIDs can be generated from any server with a simple script. Hence xgdgsgwf352sgsdesw36ff.I.35 is an absolute electronic reference. I am going to test out XMLnamespace WUIDs when I can to see whether this throws any errors through the testing app.