Edge, both in its UWP form and the new Chromium-based version, has supported dark mode for some time, but it only applies to the browser interface itself, and not web content. However, as spotted by eagle-eyed Reddit user Leopeva64-2 (via OnMSFT), Microsoft has added a flag in the latest Canary build of Edge that lets users force web content to be displayed in dark mode.
Many websites (like Neowin) already offer a built-in dark theme, and there's also a number of extensions that can force websites to display in dark mode, but it's still interesting to see the feature being implemented into the browser itself. Currently, the setting is hidden in the flags page and disabled by default, but it could eventually be added to the main settings page.
Despite being an early implementation, it already offers a variety of options for forcing dark mode, which could help adjust the effect to be more pleasant on the eyes. These include HSL-, RGB-, CIELAB-based inversion, selective image inversion, and so on. Of course, it's still a workaround for websites that don't support it, and it won't be perfect.
If you're interested in enabling the feature, you'll need to have Edge version 80.0.317.1 and go to edge://flags to find the "Force Dark Mode for Web Contents" flag.