[css4-images] element() behavior

I'm a bit puzzled by this statement in the spec [1] (highlight is mine):

Implementations may either re-use existing bitmap data generated for the
referenced element or regenerate the display of the element to maximize
quality at the image's size (for example, if the implementation detects
that the referenced element is an SVG fragment); in the latter case, the
layout of the referenced element in the image must not be changed by the
regeneration process. *That is, the image must look identical to the
referenced element, modulo rasterization quality.*


Does this mean that if the element has alpha from an ancestor, the
element() will generate a bitmap with that alpha, or is the alpha ignored
or blended?
What if an ancestor has a css filter (or blending/compositing), would the
element() return part of the filtered bitmap?
What if one of your child elements has alpha and is alpha blending with an
ancestor. Will element() return a bitmap with alpha, or a blended image?

I can see the intent of the spec, but it seems hard to implement.
Maybe you could say that element() can only reference elements that
establish a context or elements that don't contain another context.
The browser could then use the rasterized image of the context or raster
the element at that point. This might be easier to define and certainly
implement.

1: http://dev.w3.org/csswg/css4-images/#element-notation