I do not know of any, and the official list of implementations doesn't include one either.
An alternative - of course less performant than a native implementation - would be XQIB which is an XQuery implementation in JavaScript. XPath 2.0 is fully included as a subset in XQuery 1.0, so you will be able to use all XPath 2.0 features (and more) in all browsers with JavaScript support.
A short sample taken from their website on how to use it:
<script type="text/javascript" src="mxqueryjs/mxqueryjs.nocache.js"></script>
<script type="application/xquery">
b:alert(
let $x := <a><b>2</b><c>4</c></a>
return xs:string($x/b * $x/c)
)
</script>
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