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chromium/third_party/blink/web_tests/fast/multicol/dynamic/block-becomes-spanner.html 

<!DOCTYPE html>
<script>
    onload = function() {
        var elm = document.getElementById('elm');
        document.body.offsetTop; // trigger layout
        elm.style.webkitColumnSpan = 'all';
    }
</script>
<p>Test that a regular block can become a spanner.</p>
<p>Below there should be a blue box, then a yellow box, then a blue box again.</p>
<div style="-webkit-columns:2; -webkit-column-gap:0; line-height:2em; background:blue;">
    <div id="elm" style="margin:4em 0; background:yellow;"><br><br></div>
</div>