It would be nice if this article could clarify to what extent its guidelines apply to CSS outside of the Mozilla UI (i.e. on web pages). This article is being referenced by other articles in support of the idea that the CSS engine used to render web pages looks right to left through CSS selectors, but the article itself is talking about the Mozilla UI, not web pages. One would think that it probably applies to web pages too, but the article does not say so.
Regarding the appropriateness of using additional selectors on elements selected by ID (i.e.:
form#Form1), there are instances where a front-end developer has no control over markup and program-generated styling. By using additional selectors, there is a higher specificity value allowing for property overwrites. Perhaps this should also be mentioned as an exception?