用纯CSS3完成网页页面中普遍的小箭头

/* css33角形（向上 ▲） */
div.arrow-up {
 width:0px;
 height:0px;
border-left:5px solid transparent;  /* 右全透明 */
 border-right:5px solid transparent; /*右全透明 */
  border-bottom:5px solid #2f2f2f; /* 界定底部色调 */
  font-size:0px;
  line-height:0px;
}
/* css33角形（向下 ▼） */
div.arrow-down {
  width:0px;
  height:0px;
  border-left:5px solid transparent;
  border-right:5px solid transparent;
  border-top:5px solid #2f2f2f;
  font-size:0px;
  line-height:0px;
}
/* css33角形（向左） */
div.arrow-left {
  width:0px;
  height:0px;
  border-bottom:5px solid transparent;  /* left arrow slant */
  border-top:5px solid transparent; /* right arrow slant */
  border-right:5px solid #2f2f2f; /* bottom, add background color here */
  font-size:0px;
  line-height:0px;
}
/* css33角形（向右） */
div.arrow-rightright {
  width:0px;
  height:0px;
  border-bottom:5px solid transparent;  /* left arrow slant */
  border-top:5px solid transparent; /* right arrow slant */
  border-left:5px solid #2f2f2f; /* bottom, add background color here */
  font-size:0px;
  line-height:0px;
}