Unity WebGL is a build option for exporting Unity projects as JavaScript programs which use HTML5, WebAssembly, WebGL and other web standards to run Unity content in a web browser.
What is Unity WebGL?
The WebGL build option allows Unity to publish content as JavaScript programs which use HTML5/JavaScript, WebAssembly, WebGL rendering API and other web standards to run Unity content in a web browser.
As of Unity 5.4, Unity WebGL is the official replacement for the Unity Web Player.