The use of id's and classes for DOM speed

using-classes.css

/* -------------- Customer Success Stories - Sub-Layouts ----------------------- */

div.success_section { padding:0 0 20px 0; margin:0 0 20px 0; border-bottom:1px solid #dfdfdf; }
div.success_section:last-child { border:none; } 

div.success_section h2 { margin:0 0 40px 0; }
div.success_section a.all_products_link { display:block; margin:25px 0; font-size:14px; color:#ee3124; font-weight:bold; }
div.success_section a.view_all { clear:both; float:right; font-size:11px; text-align:right; color:#3b87de; }

div.success_block div.pic { margin:0 10px 20px 0; width:210px; float:left; }
div.success_block div.details { width:425px; float:left; margin:0 0 20px 0; }
div.success_block div.details p { margin:0 0 20px 0; padding:0; font-size:14px; line-height:normal; }


/* -------------- Executive Bios - Sub-Layouts ----------------------- */

.exec_bios .photo_block { min-height:265px; }
.bio_details { margin:0 0 25px 0; }
.bio_details h2 { margin:0; padding:0; font-size:24px; line-height:normal; }
.bio_details img.pull-left { margin:0 20px 15px 0; }

using-id.css

/* -------------- Follow the Aero - Sub-Layouts ----------------------- */
#sticky ul#archives li.view_all { margin:30px 0 0;}
#sticky ul#archives { border-bottom:1px solid #dfdfdf; padding:0 0 40px; width:240px; }

#featured_contributors li { margin:0 0 10px; }
#featured_contributors li a img { float:left; margin:0 10px 0 0; }
#featured_contributors li p { font-size:14px; font-weight:normal; margin: 0 0 5px 0;}
#featured_contributors li p a { font-size:14px; font-weight:normal; }

#blog_paging { margin:60px auto; width:300px; }
#blog_paging ul { -webkit-border-radius:17px; -moz-border-radius:17px; border-radius:17px;}
#blog_paging ul>li:first-child>a {border-left-width:1px;-webkit-border-bottom-left-radius:17px;border-bottom-left-radius:17px;-webkit-border-top-left-radius:17px;border-top-left-radius:17px;-moz-border-radius-bottomleft:17px;-moz-border-radius-topleft:17px}
#blog_paging ul>li:last-child>a {-webkit-border-top-right-radius:17px;border-top-right-radius:17px;-webkit-border-bottom-right-radius:17px;border-bottom-right-radius:17px;-moz-border-radius-topright:17px;-moz-border-radius-bottomright:17px}
#blog_paging ul li a { color:#ee3124; }
#blog_paging ul .active a { background-color:#ee3124; color:#FFF; }
#blog_paging ul a:hover, a:focus {background-color:#ecebec; }

/* -------------- Follow the Aero - Business Aviation Details - Sub-Layouts ----------------------- */
#blog_details h2 { font-size:24px; }
#blog_details img.blog_details_main { margin:30px 0 20px; }
#blog_details blockquote { float:right; width:330px; font-size:20px; line-height:28px; font-family:Arial, Helvetica, sans-serif; border-top:10px solid #eaeaea; border-bottom:10px solid #eaeaea; border-left:none; padding:40px 0; font-weight:100; }
#blog_details p.tags { margin:40px 0; }
#blog_details img.author_thumb { float:left; margin:0 20px 0 0; }
#blog_details h4 { font-size:16px; }
#blog_details h4 a { font-weight:normal; }
#blog_details .small_paragraph { font-size:12px; line-height:16px; }
#blog_details #blog_comments { margin:50px 0 60px; border-top:1px solid #dfdfdf; }