用纯CSS3实现网页中常见的小箭头

/* css3三角形（向上 ▲） */div.arrow-up { width:0px; height:0px;border-left:5px solid transparent;  /* 右透明 */ border-right:5px solid transparent; /*右透明 */  border-bottom:5px solid #2f2f2f; /* 定义底部颜色 */  font-size:0px;  line-height:0px;}/* css3三角形（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;}/* css3三角形（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;}/* css3三角形（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;}