Einige Tips für performante CSS Selektoren, auch wenn der Unterschied kaum wahrnehmbar ist, schadet es ja nicht darauf zu achten wenn es nicht weh tut 
- Avoid Universal Rules
- Don’t qualify ID Rules with tag names or classes
- Don’t qualify Class Rules with tag names
- Use the most specific category possible
- Avoid the descendant selector
- Tag Category rules should never contain a child selector
- Question all usages of the child selector
- Rely on inheritance
- Use scoped stylesheets
Ist wie gesagt eher für neue Webseiten, da kann man das ja auch gleich “richtig” machen, denn:
Selector efficiency and inefficiency probably isn’t going to impact most sites all that much.
We’re not talking a difference of several seconds from most to least efficient. We’re talking about milliseconds in the worst case. For the majority of sites you aren’t going to notice a difference and the effort likely outweighs the savings.