[css-properties-and-values] Setting/registering is allowed to be order-dependent

[tabatkins]

Per Seattle resolution (resolution is badly worded, read up to figure out what was actually being discussed), we're OK with an order-dependence between setting a value and registering a value.

That is, we want the following behavior:

CSS.registerProperty("--foo", { grammar: "<color>" });
el.styleMap.set("--foo", "red");
el.styleMap.set("--foo", "hello"); // throws an error
// --foo is still set to "red"

and

el.styleMap.set("--foo", "red");
el.styleMap.set("--foo", "hello"); // valid at this point; --foo is untyped
CSS.registerProperty("--foo", { grammar: "<color>" }); // valid
// because --foo was set to a non-<color> value,
// it's now set to "unset", the bottom value for properties

[tabatkins]

Further clarification: per Seattle resolution, when using a typed property in a stylesheet, setting to an invalid value doesn't trigger fallback. That is:

<script>
CSS.registerProperty("--foo", {grammar: "<color>"});
</script>
<style>
div { --foo: red; }
div.foo {  --foo: hello; }
</style>

In here, the "--foo: hello" is not thrown out at parse-time; it's accepted, but because it's invalid, it gets treated as "--foo: unset;".

• This is an intentional break from "real" CSS properties, because the threat model is different (unpredictable browsers with varying support, vs known libraries with well-known support).
• This removes the order-dependence between setting and registration when you're setting in a stylesheet, rather than thru the JS API.
• This makes typed custom properties less consistent with "real" CSS properties, but more consistent with untyped custom properties.

[shans]

What specifically needs to change in Typed OM for this? Seems like mainly a properties & values thing?