raw esi tag creates broken html

[crccheck]

I like to turn ESI off in development because of reasons. But today, I discovered that it's causing my HTML to break. The issue I'm having is that with the html5 doctype, the rendering engines are not auto-closing the esi tag. So

<esi:include src="/teh/poniez.html" />
<p>This tag should be a sibling to ESI</p>

Turns into

<esi:include src="/teh/poniez.html">
    <p>This tag should be a sibling to ESI</p>
</esi>

Which significantly changes the structure. We can't change the tag, because Vanish (and presumably Akamai, et al.) expect it to be self-closing, but we could have a dummy middleware that replaced it with an html comment (e.g. <!--esi:include src="/teh/poniez.html" /-->).