Why do browsers disallow fetch of XSLT stylesheet when URL has multiple query string parameters

I have an XML resource that references an XSLT stylesheet in a processing instruction, which has an href attribute. The URL for that attribute can have one query string parameter or none. However, when the URL has more than one query string parameter the browser seems not even to fetch the stylesheet. Why does this happen?

Here's what I mean. First, I have XML/XSLT files adapted from this w3schools example, saved to local disk as scratch.xml and style.xml. I edited the XML file to add this processing instruction near the top (second line):

<?xml-stylesheet href="style.xsl" type="text/xsl"?>

I'm running a web server in this directory using:

python3 -m http.server

If I get the XML file in the browser at http://localhost:8000/scratch.xml it fetches the stylesheet and renders fine. If I change the href attribute and add one query string parameter so that it looks like this:

<?xml-stylesheet href="style.xsl?foo=1" type="text/xsl"?>

it fetches the stylesheet and renders fine. However, if I change the href attribute and add one more parameter so that it looks like this:

<?xml-stylesheet href="style.xsl?foo=1&bar=2" type="text/xsl"?>

the browser stops even trying to fetch the stylsheet. This occurs both in Chrome and in Firefox. I verified that it's not even requesting the stylesheet by sniffing the traffic:

ngrep -d lo port 8000

I find this baffling, honestly.

I've tried at least two different browsers, and I've tried at least two different web servers (nginx and the python server). I didn't expect the form of the stylesheet URL to have an bearing at all.

Your XML is not well-formed because the & character needs to be escaped as &.