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Test Info:

<!doctype html>
<title>Dynamic evaluation of media queries works fine in presence of empty media rule</title>
<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
<link rel="author" href="https://mozilla.org" title="Mozilla">
<link rel="help" href="https://drafts.csswg.org/mediaqueries-4/">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<iframe width=500 height=300 frameborder=0></iframe>
<script>
let iframe = document.querySelector("iframe");
promise_test(async function (t) {
await new Promise(resolve => {
window.addEventListener("load", resolve);
});
let frameLoaded = new Promise(resolve => {
iframe.addEventListener("load", resolve);
});
iframe.srcdoc = `
<style>
:root { background-color: red; }
/* This one should never apply */
@media (min-width: 1500px) {}
/* This one should change and start matching */
@media (max-width: 400px) {
:root { background-color: lime; }
}
</style>
`;
await frameLoaded;
function getColor() {
return iframe.contentWindow.getComputedStyle(iframe.contentDocument.documentElement).backgroundColor;
}
assert_equals(getColor(), "rgb(255, 0, 0)", "Should start red");
iframe.width = 400;
assert_equals(getColor(), "rgb(0, 255, 0)", "Should turn green");
});
</script>