This rule aims to reduce DOM access assigning its object to variable when access multiple time. It saves CPU cycles.
Examples of incorrect code for this rule:
var el1 = document.getElementById('block1').test1;
var el2 = document.getElementById('block1').test2;
Examples of correct code for this rule:
var blockElement = document.getElementById('block1');
var el1 = blockElement.test1;
var el2 = blockElement.test2;