Detailed proposal for layered fill and stroke

Hello all,

I have written up a detailed proposal for how multiple fill and stroke
paint layers could be implemented as a set of coordinating style properties:

   -
   https://www.w3.org/Graphics/SVG/WG/wiki/Proposals/Layered_Paint_Properties

Please take a look and let me know what you think.

The SVG WG has committed to allowing multiple paint layers in the fill and
stroke property, but there has not been much careful discussion of how it
would work.  The current spec copies the syntax from CSS backgrounds with
minor alterations that do not fully address the differences between paint
servers and background images, or between SVG and CSS layout.

The proposal also includes practical implementation approaches for the
following issues:

- How context fill and stroke colors can be used in both markers and paint
servers, when those properties can themselves be layered lists of paint
servers.
- How paint order of strokes, fills, and markers can be controlled when
there are many layers of paint and many possible marker types.
- How authors can specify alternative object bounding boxes for paint
servers, so that e.g. stroke paint can be scaled according to the stroke
region.

With respect to reference bounding boxes, the proposal could also provide
an easier to implement solution for clipping, masking, and filters.  Please
look over it closely if you're working on one of those FX specs.  Currently
there are significant inconsistencies in those specs with respect to how
objectBoundingBox and userSpaceOnUse units should interact with non-SVG
bounding boxes.

Best,
Amelia BR