-e: see title, new settings for standalone fullscreen got introduced and they effect WebGL too
--works in 2017.3, starts not working in 2018.1a1

-repro:
--create a new project in latest 2018.1
--add script that does:

if (Input.GetKeyDown(KeyCode.F))
{
Screen.fullScreen = !Screen.fullScreen;
}

--build & run for WebGL
--click into scene and press F
--NOTICE fullscreen isn't triggered

--Workaround:
--in Standalone Player settings switch from "Fullscreen Window" to "Exclusive Fullscreen"
--save project
--rebuild & run WebGL
--click into scene and press F
--notice fullscreen is triggered