The motherboard vendors allow regular DDR3 to work, but they're not responsible for the CPU's memory controller, so they don't really care if you burn that out, that's Intel's problem. Intel has stated that only DDR3 sticks running at 1.35V is officially supported on Skylake CPUs and anything running at a higher voltage risks damaging the memory controller.
Really though, DDR4 support is one of the big things Skylake adds, so buying an LGA 1151 board that only supports DDR3 is kind of defeats the purpose of going with Skylake, as the performance difference between Haswell and Skylake isn't that big, and the only other big difference is Skylake's improvedI/O, most of which is actually stripped out on the H110 chipset, neutralizing that advantage.
If you can afford to get DDR4, that's what I would get if I wanted to go with Skylake, I'd also move up to at least an H170 board just to get some of the I/O improvements, but that might not be a huge deal for you if you don't plan on getting a really high performance SSD or something like that. If you really need to save some money and want to keep the RAM, Haswell is still quite capable.