Selectors should be 3 or less levels

gistfile1.css

/* BAD: The following  selectors should be avoided if possible */

.one .two .three .four   { /* styles */ }
.one ul li a             { /* styles */ }
.one ul a:hover          { /* styles */ }
.one ul li:first-child   { /* styles */ }

/* BAD: Pre-processed nested selectors */

.one     {

   .two .three.four      { /* styles */ }
   ul li a               { /* styles */ }
 
}

/* GOOD: No more than 3 levels deep */

.one .four               { /* styles */ }
.one li a                { /* styles */ }
.one a:hover             { /* styles */ }
.one li:first-child      { /* styles */ }
.one a:hover             { /* styles */ }


/* ACCEPTIBLE: Discretion is advised */
/* This is 4 levels, but the extra specificity may be preferable */

.one li a:hover          { /* styles */ }
.one li a.active         { /* styles */ }