WebGL is a Web standard that allows issuing OpenGL ES 2.0-based instructions to compatible graphics cards, which is just about everything today. It has programmable vertex and fragment (pixel) shaders with a decent amount of flexibility. Engines like Unity have been looking toward using this technology as a compile target, because Web browsers are ubiquitous, relatively user friendly, and based on standards that anyone could implement should a work of art benefit from preservation.
Image Credit: Mozilla
Until Unity 5.3, this feature was in “preview” levels of support. This upcoming release, scheduled for today according to their roadmap, drops this moniker. It is now a build target with official support.
To run WebGL applications that are built in Unity, the vast majority of features target recent versions of Firefox, Chrome, and Edge for Windows 10 Version 1511. (The November Update for Windows 10 added the ability to lock the mouse cursor, which is obviously useful for mouse and keyboard titles.)
We're still a long way from web browsers being equivalent to game consoles. That said, they are catching up fast. You could easily have an experience that shames the last generation, especially when WebGL 2 lands, and you don't have to worry about what happens in 10, 40, or even hundreds of years as long as society deems your art worthy for preservation. I do hope that some artists and serious developers take real advantage of it, though. Shovelware could obscure its power and confuse users, and we know they will be pretty much first out of the gate.