Dangling CSS combinators don't trigger any errors but produce an invalid stylesheet.

[yamadapc]

Describe the bug

Dangling CSS combinators don't trigger any errors but produce an invalid stylesheet.

To Reproduce

Write code with a dangling CSS combinator, for example:

const header = css({
     '>': {
        marginLeft: 10
     }
});

Expected behavior

A build error would be triggered or the style would be ignored.

Actual behavior

A stylesheet is produced with a dangling selector:

.hash > {
    margin-left: 10px;
}