I was playing with CSS and wrote the following code:
<div id="triangle1" class="triangle"></div>
You haven't set the color for top, left and right border, so it takes the inherited color value (as no value has been set, it takes black by default). To set the other borders color just add this to the triangle styles.
border-top-color: transparent; border-left-color: transparent; border-right-color: transparent;
See demo http://codepen.io/8odoros/pen/EyLQpd