Fixed it. The proper version uses both a normalize.css and a base main.css (from the HTML5boilerplate) file, which weren’t available in the Codepen. One of them has the effect of making the <main> tag usable in IE11, which doesn’t otherwise work in the Codepen.
For the purposes of the Codepen, I’ve swapped out the <main> tag for <div class="main">
v2 here