https://arstechnica.com/gadgets/2018/02/apple-clarifies-which-audio-sources-are-supported-on-homepod-speakers/

Since Apple's HomePod became available for preorder last week, there has been confusion about the music it can play. Apple recently added an "audio sources" section to HomePod's tech spec webpage that lays out exactly where you can play music from using the smart speaker. These are the supported sources:

It's not surprising that nearly all of these sources draw music from Apple's ecosystem. HomePod was always going to prioritize Apple-approved music before all other types, but that will make it harder for users who don't dwell in Apple's ecosystem to use HomePod regularly.