12：sass中宏、继承以及占位符的比较

margin-top: 10px; }
.box1 span {
display: inline-block;
margin-top: 20px; }

.box2 {
  margin-top: 10px; }
.box2 span {
  display: inline-block;
  margin-top: 20px; }

.mt, .header1, .header1 span, .header2, .header2 span {
  margin-top: 5px; }

.header1 span {
  display: inline-block; }

.header2 span {
  display: inline-block; }

.footer1, .footer1 span, .footer2, .footer2 span {
  margin-top: 5px; }

.footer1 span {
  display: inline-block; }

.footer2 span {
  display: inline-block; }