​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;}