Can't you just set left
to 50%
and then have margin-left
set to -25px
to account for it's width: http://jsfiddle.net/9AbYc/
.hero:after {
content:'';
position: absolute;
top: 100%;
left: 50%;
margin-left: -50px;
width: 0;
height: 0;
border-top: solid 50px #e15915;
border-left: solid 50px transparent;
border-right: solid 50px transparent;
}
or if you needed a variable width you could use: http://jsfiddle.net/9AbYc/1/
.hero:after {
content:'';
position: absolute;
top: 100%;
left: 0;
right: 0;
margin: 0 auto;
width: 0;
height: 0;
border-top: solid 50px #e15915;
border-left: solid 50px transparent;
border-right: solid 50px transparent;
}