Some will, some won't. It will be just like the current console market, although with a PC you will undoubtedly be able to find some work-a-rounds to get something working that might not be supported.
Rest assured that Steam will support as many HMD's as possible (definitely the big two).
i7 5960X @ 3.8 GHz | Corsair Vengeance LPX 16GB DDR4 PC2800 | GTX Titan X Pascal | Win 10 64 bit | Asus ROG PG348Q | EVGA X99 Classified