I'm not sure that's valid or not. I just got from [atom/language-html](https://github.com/atom/language-html/blob/a4779ddc8f4d21d0ef177d6139cecd2a5b75b760/scoped-properties/language-html.cson#L8).
I'm not understand atom deeply yet and atom's document is poor now. `scoped-properties` is not even documented.
but `language-html` is atom's official package. So I trust it. I have to check again what it exactly mean.