After a recent edit in relation to relaxed nesting, css-syntax now says this:

<{-token>
If the first two non- values of rule’s prelude are an whose value starts with "--" followed by a , consume the remnants of a bad declaration from input, with nested, and return nothing.

Unless I'm mistaken, this means that encountering this situation top-level means we'll treat the entire rest of the stylesheet as a "bad declaration".

--foo:hover {
  color: red;
}
/* Lots of other innocent and valid rules here */

That is probably a bit extreme, and makes the change a bit hard to ship, since it could affects existing sites (drastically).

Perhaps we can limit the behavior to when nested is true? Or something else? @tabatkins