[css-properties-values-api] mention how animations involving invalid token stream values work

[heycam]

Imagine a transition that involves an invalid token stream value:

<style>
p { transition: --test 1s; }
p:hover { --test: something; }
</style>
<p>hover me</p>

When you hover over the <p>, it should generate a transition from the underyling value, which is the initial value -- the special invalid token stream value -- to the token stream " something". I think this should be valid, and, since it's a token stream typed custom property, and therefore not interpolable, will switch value at 0.5s.

Similarly in the other direction:

<style>
p { transition: --test 1s; --test: something; }
p:hover { --test: var(--test); }
</style>
<p>hover me</p>

Here the initial, underyling value is the token stream " something" and the end value is the special invalid token stream value.

I think it makes sense that animations involving the special invalid token stream values work, but it would be good to call this case out specially in the spec somewhere.

@birtles @jyc

[tabatkins]

The special invalid value isn't actually "invalid" - it just triggers the "invalid at computed-value time" behavior when substituted via var() (or triggers fallback). It's still a CSS value like any other, and should transition at 50% like anything else that doesn't have a smarter interpolation strategy. Why would we call this out specially from any other arbitrary value?

[heycam]

OK, maybe my mental model of the special value was slightly askew. You might want to call it out in a note, just for people like me who thought it might require special casing. But up to you.

[tabatkins]

Happy to do so.