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chromium/third_party/blink/web_tests/fast/reflections/reflection-masks-outset.html 

<!DOCTYPE html>
<div id="target"
    style="width: 100px; height: 100px; background: blue;
           margin:20px; outline:20px solid green;
           -webkit-box-reflect: below 2px url(resources/vignette-mask.png) 75 / auto / 25px stretch">