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Test Info:

<!DOCTYPE html>
<!-- Any copyright is dedicated to the Public Domain.
<html>
<style>
details {
column-count: 3;
column-rule: 1px solid lightgray;
-webkit-column-count: 3;
-webkit-column-rule: 1px solid lightgray;
border: 1px solid lightblue;
}
summary {
background-color: lightgreen;
}
</style>
<body>
<details open>
<summary>Summary</summary>
<p>line</p>
<p>line</p>
<p>line</p>
<p>line</p>
<p>line</p>
<p>line</p>
<p>line</p>
</details>
</body>
</html>