.class1 .class2 h2{
    color: red;
}

<h2>

.class2

.class1

div p {
  background-color: yellow;
}

<p>

<div>

<i style="display:block;clear:both"></i>

.clearfix::after {
  content: "";
  clear: both;
  display: block;
}

.main-inner div

box-sizing: border-box

#nav li {
    list-style: none;
    float: left;
    padding: 0 15px 0 0;
}

#body {
    background: url(../images/bg.jpg) top left repeat-y;
}

span

a

display: inline-block;

display: block;

#header h1 {
    background: url(../images/bg.jpg) top left no-repeat;
    width: 702px;
    height: 100px;
    text-indent: -9999px;
}