Restricted JS Execution Contexts

Various Web platform features have a need for script to run in "restricted contexts", where some features accessible to regular main-thread page JS are not available.

Audio Workers

Web Audio wants an "audio worker" --- processing audio samples on a real-time audio thread. Hopefully small snippets of code doing small amounts of work on each callback (e.g. processing 128 audio samples).

Vertex Shaders

For CSS Filter-ish use-cases, we want to be able to generate meshes by applying a JS function to vertices of a grid, mapping 2D points to 2D points.

CSS Style Callbacks

To extend the CSS style system, we want to provide callbacks that run during style computation to parse values and generate computed style values.

CSS Layout Callbacks

To extend CSS layout, we want to provide callbacks that run during layout, to layout the contents of container elements.

CSS Painting Callbacks

To extend CSS painting, we want to provide callbacks that run during painting, to paint the contents of elements.

API	Work per callback	Retain state between callbacks?	Isolated worlds per page	Send/receive messages	Other Web platform APIs needed
Audio workers	Very little	Yes	dozens	Yes	Minima
Vertex shaders	Very little	No	flexible	No	Minima
CSS style extensions	Very little	No	lots	No	Minimal
CSS layout extensions	Lots (e.g. constraint solving)	Yes (caching)	potentially lots	No	Minimal
CSS painting extensions	Potentially a lot	probably not	dozens	No	Canvas