CSSで三角矢印を描く

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