bootstrap css => h1-6

定义了标题H1-6 的初始样式
定义了 .H1-6 类
定义了副标题 =>H1-6 small
small 在H1-6主标题或H1-6类下当作副标题

h1,h2,h3,h4,h5,h6,
.h1,.h2,.h3,.h4,.h5,.h6 {
  font-family: inherit;
  font-weight: 500;
  line-height: 1.1;
  color: inherit;
}
small,.small {
    font-size: 85%;
}
h1 small,h2 small,h3 small,h4 small,h5 small,h6 small,
.h1 small,.h2 small,.h3 small,.h4 small,.h5 small,.h6 small,
h1 .small,h2 .small,h3 .small,h4 .small,h5 .small,h6 .small,
.h1 .small,.h2 .small,.h3 .small,.h4 .small,.h5 .small,.h6 .small {
  font-weight: normal;
  line-height: 1;
  color: #777;
}
h1,.h1,
h2,.h2,
h3,.h3 {
  margin-top: 20px;
  margin-bottom: 10px;
}
h4,.h4,
h5,.h5,
h6,.h6 {
  margin-top: 10px;
  margin-bottom: 10px;
}
h1 small,.h1 small,h2 small,.h2 small,h3 small,.h3 small,h1 .small,.h1 .small,h2 .small,.h2 .small,h3 .small,.h3 .small {
  font-size: 65%;
}
h4 small,.h4 small,h5 small,.h5 small,h6 small,.h6 small,h4 .small,.h4 .small,h5 .small,.h5 .small,h6 .small,.h6 .small {
  font-size: 75%;
}
h1,.h1 {
  font-size: 36px;
}
h2,.h2 {
  font-size: 30px;
}
h3,.h3 {
  font-size: 24px;
}
h4,.h4 {
  font-size: 18px;
}
h5,.h5 {
  font-size: 14px;
}
h6,.h6 {
  font-size: 12px;
}