Why does .css('fontSize') produce different results in Edge?

Question

Consider this code (also in a fiddle):

document.getElementById("span").innerHTML += $('#input').css('fontSize');

span input {
  font-size: inherit;
}

input {
  font-size: 15px;
}

<span id="span" style="font-size: 30px;">
  <input id="input"/>
</span>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>

In Chrome and Firefox, the .css('fontSize') will return 30px, in Edge and IE it's 15px. Why does it do that? The DOM Explorer in Edge even shows the 15px in strikethrough, and therefore should take the inherited 30px as the fontSize:

And the input is rendered with a 30px font, so IE/Edge is picking it up for rendering purposes.

Answer 1

Update: The bug below is now fixed; FremyCompany says he/she is a program manager from the Edge team and the fix will reach customers in early 2017.

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It looks very much like an IE and Edge bug. Not having found it, I reported it.

Here's an update to the snippet that sees what IE/Edge is telling jQuery via getComputedStyle or currentStyle:

var input = $("#input");
console.log("jQuery: " + input.css('fontSize'));
if (window.getComputedStyle) {
  console.log("getComputedStyle: " + getComputedStyle(input[0]).fontSize);
}
if (input[0].currentStyle) {
  console.log("currentStyle: " + input[0].currentStyle.fontSize);
}

span input {
  font-size: inherit;
}

input {
  font-size: 15px;
}

<span id="span" style="font-size: 30px;">
  <input id="input"/>
  <span id="size"></span>
</span>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>

For me, IE11 returns 15px from both getComputedStyle and the Microsoft-specific currentStyle property (it's reassuring that they do at least say the same thing):

So it's not a jQuery bug, it's a Microsoft bug when reporting the size via JavaScript (looks like when inherit is the governing rule), even though it's rendering it correctly.

I tried to find a way to make this a grey area, but couldn't think of anything. For instance, according to the spec, having an input inside a span is entirely valid.