Forward-compatibility escape hatch for engine-specific options.
OptionalfillFill colour (CSS colour string).
OptionalfillFill opacity. Range: 0 … 1.
OptionalcolorStroke colour (CSS colour string).
OptionalweightStroke width in pixels.
OptionalopacityStroke opacity. Range: 0 … 1.
OptionaldashStroke dash pattern (SVG stroke-dasharray format, e.g. "5, 10").
OptionallineSVG stroke-linecap value ("butt", "round", "square").
OptionallineSVG stroke-linejoin value ("miter", "round", "bevel").
Vector fill and stroke style options, engine-agnostic.
The index signature allows adapters to accept engine-specific style extensions without breaking the contract.