[whatwg] Outline style to use for drawSystemFocusRing

[Dominic Mazzoni]

I'm looking into implementing
drawSystemFocusRing<http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#dom-context-2d-drawsystemfocusring>in
Blink, but it's unclear what style to use when drawing the focus ring
-
i.e. the color, thickness, etc., assuming that the user hasn't overridden
it for accessibility. Here are some possible interpretations:

1. Hard-code default values. I don't think this would be ideal, since it
wouldn't necessarily match the focus color of other elements that get the
focus color from the system.
2. Get them from the default style of a generic element (which may match
the system focus style). This could work, but I think this is less flexible
for developers - if they picked a custom outline color for every other
element in their site but would have no way to override it for canvas
(other than using drawCustomFocusRing).
3. Get them from the outline style of the canvas element. This means every
focus ring within the canvas would be the same, but otherwise I have no
objection.
4. Get them from the outline style ofelement passed to drawSystemFocusRing.
This is my preference.

- Dominic