The Specificity Score is calculated by the browser to dertermine which CSS rules can over-ride which others.

Calculate the selector specificity to determine which rules have the highest scores.
Higher specificity scores will always over-ride lower specificity.
If score is equal, then sort order applies: rules that appear later can over-ride rules that appear earlier.

Make three tallies for each occurrence of:
I	- #id selectors	
C	- .class, :pseudoclass, []attribute selectors
E	- HTML element selectors

CSS Selector									I	C	E	Specificity Score 
-----------------------------------------------------------------------------
li												
.myclass										
#someid											
section > article ul li							
.myclass table th ol li							
aside #someid .myclass							
aside[id] ul li:last-of-type					
.myclass[class*="someotherclass"] > ul ol li	
nav ul li:hover:nth-child(2n) 					
*												

Want more?
https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity
https://stuffandnonsense.co.uk/archives/css_specificity_wars.html