The density mark could support the density keyword for fillOpacity and strokeOpacity, too

[mbostock]

It’s currently only supported for stroke and fill:

plot/src/marks/density.js

Lines 24 to 29 in 02731fb

	// If fill or stroke is specified as “density”, then temporarily treat these
	// as a literal color when computing defaults and maybeZ; below, we’ll unset
	// these constant colors back to undefined since they will instead be
	// populated by a channel generated by the initializer.
	const fillDensity = isDensity(fill) && ((fill = "currentColor"), true);
	const strokeDensity = isDensity(stroke) && ((stroke = "currentColor"), true);

[Fil]

For fillOpacity the tricky thing is that the polygons are layered (as opposed to what we would have with isobands), so the opacities will be compounded. d3/d3-contour#71

[mbostock]

Agreed. I think we should switch to generating isobands by default.

[Fil]

Generalized as #1420.

[mbostock]

And strokeWidth.