Tests that ::backdrop inherits from its originating element
Dumping matched rules: 
*dialog::backdrop* {    regular
    opacity: 0.5; @[9:4-9:17]
    opacity: 0.5; @[undefined-undefined]
}
Dumping inherited rules: 
*dialog* {    regular
    color: green; @[6:4-6:17]
    color: green; @[undefined-undefined]
}
*:root* {    regular
    --foo: bar; @[3:4-3:15]
    --foo: bar; @[undefined-undefined]
}