You can soften the effect if different parties to indicate slightly different values for the color. Superimposition with a translucent boarders only happens if they are the same color. Therefore, if you specify all 4 sides do not differ greatly in color (with a transparency value different by 0.01), the superimposition is not so noticeable.
\r
\r
div { margin:40px 0 0 40px; border:30px solid; border-color:rgba(0, 0, 0, 0.5) rgba(0, 0, 0, 0.49) rgba(0, 0, 0, 0.51) rgba(0, 0, 0, 0.52); width:500px; height:500px; background:#ddd; padding:30px; white-space:pre; }
\r
P. S. About it there are a few bugs on
https://bugs.webkit.org but they are all open.