In DPS folios, a lot of publishers used the HTMLResources.zip file as a way to link to PDF files so that tapping a button would display the PDF file in an in-app browser. At this time, neither shared HTML resources nor linking to PDF files is supported in DPS 2015. But as Neil pointed out, InDesign-based HTML overlays work just fine in DPS 2015.
Forgive my confusion: Does this mean that for HTML overlays, the content must be hosted somewhere and is loaded remotely, instead of contained in the HTMLResources.zip file and available even when the app is used without an internet connection?
Like I say, I may be missing something basic about the workflow.
Sorry--brain fart. I was thinking all of the supporting files were in the HTMLResources.zip. Forgot that they just had to be in the folder with the index.html file.