[css-properties-values-api] Define effects on CSSOM.

[andruud]

• Define how setProperty() behaves for registered custom properties.
• Define how values reify.
• How underlying values are created is not defined, as I think the
  current description in css-typed-om covers it.

[andruud]

This is the final missing "substantial" piece of this spec, I think.

Not sure if it's OK to use <declaration-value> in this way. CSS Typed OM seems to have a concept of underlying values "being" a certain type (rather than e.g. matching a production), in which case it should be fine.

[tabatkins]

Did some minor tweaking to wording and linking, but nothing to the algos. Looks good otherwise, sorry for the delay in reviewing.

CSS Typed OM seems to have a concept of underlying values "being" a certain type (rather than e.g. matching a production), in which case it should be fine.

Yeah, underlying values are an implementation-defined structure, that just must be able to reproduce the necessary data for reification/stringification. What you have down is fine; the only thing that's not a <declaration-value> but still a valid custom property value is the empty value, and that can't ever match a syntax, so it's fine to let that fall into the second case.

[andruud]

Thanks Tab 👍