[compositing] Various small fixes for Compositing & Blending Level 1

1) Examples and figures don't match

A number of the examples in the Compositing & Blending Level 1 spec use the
color keyword `green` (#080) in the code, but the figures are clearly using
`lime` (#0F0):

   - Example 1 (either the code or the figure could be changed)
   - Example 2 (to get the primary/secondary color effect, you really need
   to use #0F0)

Also for Example 2, to create the figure as shown, the code should include

svg { isolation: isolate; }

As it is, screen mode applied to the default white page background results
in all white.  (If browsers are supposed to treat the root element canvas
as transparent black, then that needs to be stated somewhere and
implementations need to update their behavior.)

Alternately, since Example 2 comes before the discussion of the `isolation`
property, it might be easier to simply add the rule

svg { background-color: black }


And update the figure accordingly.

See http://fiddle.jshell.net/cghv8k0j/ in a browser that implements the
blending spec.

2) There is some mangled markup in the spec.

The most serious effect is that the `background-blend-mode` property is not
being correctly listed in the table of contents.  This is showing up in
both the Editor's Draft and the published spec.

3)  `background-blend-mode` is a CSS property, not only for HTML

The table for  `background-blend-mode`  says "Applies to:All HTML
elements".  However, it could apply to any XML content that uses a CSS
layout model.  That includes a top-level inline SVG element; in practice
(and probably in SVG 2) it would also include a root <svg> element.

A more useful and future-proof description would be "Applies to: Any
element that renders the `background-image` property".  Another way to make
the same distinction is to use the language from the Transforms spec
is "elements with (or without) associated CSS layout box".


--AmeliaBR