The upcoming new version of macOS (Mojave) will have a dark mode option. While this is an option on the operating system level and will affect apps, it will also affect websites viewed in Safari turning the default background color of
<html> to a dark gray. Here is a tweet by Kuldar Kalvik showing a screenshot of it.
Although this behaviour might change until the official release of macOS Mojave I still think this is a good reminder to always set a background color for
<html> in your CSS. Almost all browsers default to white, but it’s not an industry wide defined standard as far as I know.