Minecraft 18w10a, Minecraft 18w10b, Minecraft 18w10c, Minecraft 18w11a, Minecraft 18w14a, Minecraft 18w14b
Stand in double tall grass and look at the light level using F3.
Tall grass seems to be darker than regular grass, for reasons unknown. Screenshots comparing 1.12.2 and 18w10b attached.
MC-123853 Mobs don't burn when standing in double plants/cobweb