​CSS如何写出三角形

/* create an arrow that points up */ div.arrow-up {  width:0px;  height:0px;  border-left:5px solid transparent; /* left arrow slant */  border-right:5px solid transparent; /* right arrow slant */  border-bottom:5px solid #2f2f2f; /* bottom, add background color here */  font-size:0px;  line-height:0px; }  /* create an arrow that points down */ 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; }  /* create an arrow that points left */ 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; }  /* create an arrow that points right */ div.arrow-right {  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; }