Simon Kornblith wrote:
MLA requires a double-spaced bibliography while most other styles use
a single-spaced bibliography. In addition, some styles do not place a
line between references, while others do. Both of these should be
options.
To play devil’s advocate, I’d say this should be the domain of the main
document styles and templates, and so out of scope for CSL.
In truth, the distinction between document styles and citation styles
are rather artificial (since style guides cover both), so we just need
to make a decision about where we want the line to be.
To get to practical implementation issues, I’ve never liked that Zotero
doesn’t tag the bibliography with a style, and does all formatting
locally. I typically have my main document template pre-set with a style
to configure this stuff. So my workflow when using Zotero is to insert
the bibliography, then to tag it with the “bibliography” style. If
Zotero did this ahead of time, I’d not have to do that, and the spacing
would already be correctly established.
WRT to your specific suggestions:
For double-spacing the entire bibliography, we definitely need:
attribute line-spacing { xsd:decimal }
where the default would be 1.
So, for example, 1 1/2 spacing would be with a value of “1.5”?
I’m not sure about how to handle spacing between references. I’d
imagine Bruce has some objections to a simple “delimiter” attribute
to configure this, since using
tags in HTML is considered
harmful.
Indeed 
Perhaps just:
attribute entry-spacing { xsd:decimal }
That would be fine (if you convince me my devil’s advocate position is
wrong).
Bruce