[css-variables] Unable to do feature matching at parse time when using var() #1348
Description
When using var() references you can no longer rely on parse time failure to automatically match on supported syntax/features.
See example from user bug report: https://bugs.chromium.org/p/chromium/issues/detail?id=700445#c13
--x1: url(data:image/png;base64,<PNG>);
--x2: url(data:image/png;base64,<PNG>);
background-image: var(--x1);
background-image: image-set(var(--x1) 1x, var(--x2) 2x);
One workaround for this use case is to use @supports. Is this a desirable state of the API or should the fallback behaviour that used to happen at parse time be brought to variable resolution in some form?