If you have an element with multiple border colors and a border radius specified, the border will generally disappear when using pinch-to-zoom. The border will be displayed while zooming, but once releasing, it will disappear. This does not happen when using the regular page zoom tool (cmd+/cmd-). When using pinch to zoom to return to 100% zoom, the border will reappear.
The attached test case should reproduce this. The "one color" entries will always be displayed; the "two color" entry without a radius will always be displayed. The final "two color"+radius entry will disappear after pinch to zoom.
This has been seen in multiple iOS versions and the current Safari nightly on OS X.
Created attachment 237150 [details]
Bug test case