Setting your height and border-thickness in pt or px (I know, I know... but this is *not* for setting text!) should give more consistent appearance. Anyway, that's how it seems to work for me.
Setting it in em will vary with font-size.
Code:
hr, .hr100
{
border-top: 2pt solid blue;
border-left: none;
border-right: none;
border-bottom: none;
height: 2pt;
margin-top: 1em;
margin-bottom: 1em;
margin-left:0;
margin-right:0;
}