This WebGL extension, exposing the underlying OpenGL ES functionality, allows developers to measure the duration of the commands they submit to the graphics processor. This will allow the development of additional tools to help developers make their WebGL code run faster.
Docs: Conformance test: https://www.khronos.org/registry/webgl/sdk/tests/conformance/extensions/ext-disjoint-timer-query.html (Open-sourced in Github repository at https://github.com/KhronosGroup/WebGL )