The specification discusses specificity under the section on the nesting selector (&):

The specificity of the nesting selector is equal to the largest specificity among the complex selectors in the parent style rule’s selector list (identical to the behavior of :is()).

However, it doesn't seem to directly address specificity for nested selectors that have multiple parent selectors but do not use &.

.foo,
#foo {
  .bar { ... }
}

Is the nested selector here also equivalent to :is(.foo, #foo) .bar? (or, in other words, is a leading & implicit in this example?)

The current implementations in both Chrome and Safari seem to interpret it that way (demo), and I think this probably makes sense from an implementation standpoint, but I'm having trouble finding something concrete in the spec that states this one way or the other.