<!DOCTYPE html>
<html>
<head>
<script>
/*
user_pref("layout.css.grid.enabled", true);
*/
function boom()
{
document.getElementById("r").appendChild(document.createTextNode("B"));
}
</script>
</head>
<body onload="boom();">
<div id="r" style="display: grid;">A</div>
</body>
</html>