My attempt to use background-clip: content-box is failing. The entire box has a background, including the padding area, not just the content area.
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The background CSS property is a one-line way to define all background properties, including background-clip. Thus, specifying background: #ffffd unsets your earlier setting of background-clip. If instead you replace that line with background-color: #ffffd, it should work the way you want it to.
Here's the small modification to your jsfiddle.