Re: [w3ctag/spec-reviews] Custom Paint API to CR (#140)

Some stream-of-consciousness notes:

 - Overall this looks great! A solid addition to the platform. Great to see it.
 - I like the use of ES6 classes for defining worklet code. Is an ES5 syntax also supported? If not, why not?
 - None of the examples show importing the worklet code. This seems like a major omission.
 - The implications for, e.g., drawing shadows outside the main margin box, aren't clear from the prose
 - The spec should link to it's companion [explainer](https://github.com/w3c/css-houdini-drafts/blob/master/css-paint-api/EXPLAINER.md) at the top.
 - This spec normatively references the WHATWG HTML canvas API (e.g. [this](https://html.spec.whatwg.org/multipage/scripting.html#canvasstate)). That spec references early drafts from task forces (e.g. [this](https://drafts.fxtf.org/compositing/)). Worth at least discussing.
 - Why does the `PaintRenderingContext2D` interface not implement `CanvasFilters`? This deprives custom paint of many interesting effects vs. a Shadow DOM with a `<canvas>` background image
 - The [Image Placeholder](https://drafts.css-houdini.org/css-paint-api/#example-2) example is seems unsatisfying. How can a developer fade the loaded image in? Or implement any other sort of transition over time?
 - Related: how does custom paint interact with timelines; e.g. the Web Animation API?
 - The [conic gradient example is non-existent](https://drafts.css-houdini.org/css-paint-api/#example-3).
 - Why isn't `CanvasText` supported?
 
Regards

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