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var im = document.getElementsByTagName("img");
for (var i = 0, l = im.length; i < l; i++) {
im[i].src = im[i].getAttribute('src').replace(/\.svg$/, '.png');
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}
</script>
<img src="firefox-logo.png">
<script>
if (
document.implementation.hasFeature("http://www.w3.org/TR/SVG11/feature#Image", "1.1") &&
"querySelectorAll" in document
) {
var im = document.querySelectorAll('img[src$=".png"]');
for (var i = 0, l = im.length; i < l; i++) {
im[i].src = im[i].getAttribute('src').replace(/\.png$/, '.svg');
}
}
</script>