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Reported also here: https://bugzilla.mozilla.org/show_bug.cgi?id=1828390

Heya, yep this is consistent with the CommonMark spec:

• https://spec.commonmark.org/0.30/#example-502
• https://spec.commonmark.org/dingus/?text=%5Bexample%5D%5BEXAMPLE%5D%0A%0A%5BEXAMPLE%5D%3A%20https%3A%2F%2Fexample.com%3Ffirst%3Da%26second%3Db%20%22Example%22%0A

and also I believe with the HTML spec: https://stackoverflow.com/a/3705601/5033292

are you sure it is actually breaking the link, it doesn't appear to break for me in Firefox?

It will load the page but the query param isn't properly sent to the server, so links like this turn into this and don't process the start time.

What you said isn't quite correct; it turns it into <a href="https://www.youtube.com/watch?v=dQw4w9WgXcQ&amp;t=2m12s">this</a> (not with the backslash escapes you put in your 2nd link above), which renders as this
and any browser I try this link on correctly processes the start time (and gives me a musical surprise lol)

@chrisjsewell yeah, the backslash escapes were for GitHub rendering to make a link that matches when you click it (if you look at the resulting href on GitHub it is accurate)

Although testing on my phone just now it works as I would want it to, but that’s with the YouTube app handling it, not a browser

I am having the exact same issue. For https://example.com?first=a&second=b, the api service I am using complains that the second parameter is amp;second, which it expects to be second. Is there any way to let myst not encode the url?

Update: I ended up manually using sed to find and remove the amp;.

Same here. Markdown-it-py 3.0.0 generates the correct url with & encoded as &

$ markdown-it <(echo "[example](https://example.com?first=a&second=b)")
<p><a href="https://example.com?first=a&amp;second=b">example</a></p>

whereas myst-parser 2.0.0 yields a broken &

$ myst-docutils-demo <(echo "[example](https://example.com?first=a&second=b)")
<p><a class="reference external" href="https://example.com?first=a&amp;amp;second=b">example</a></p>

A potential solution would be to adjust the MyST-Parser code to prevent the initial escaping of the URL, allowing docutils or the final HTML writer to handle the necessary escaping. This could involve simply removing or modifying the call to escapeHtml in the MyST-Parser's conversion process.

diff --git a/myst_parser/mdit_to_docutils/base.py b/myst_parser/mdit_to_docutils/base.py
index 4a02459..cee67c9 100644
--- a/myst_parser/mdit_to_docutils/base.py
+++ b/myst_parser/mdit_to_docutils/base.py
@@ -996,7 +996,7 @@ class DocutilsRenderer(RendererProtocol):
             if "classes" in conversion:
                 ref_node["classes"].extend(conversion["classes"])
 
-        ref_node["refuri"] = escapeHtml(uri)
+        ref_node["refuri"] = uri
         if implicit_text is not None:
             with self.current_node_context(ref_node, append=True):
                 self.current_node.append(nodes.Text(implicit_text))

Additional Information:

The issue seems to stem from both the MyST-Parser and docutils/html writers, where the URL is being escaped more than once.
https://github.com/docutils/docutils/blob/b768e2626088711dec257b0847b563d02700a712/docutils/docutils/writers/_html_base.py

$ echo "http://a.com?foo=1&bar=2" | rst2html.py | grep foo
<p><a class="reference external" href="http://a.com?foo=1&amp;bar=2">http://a.com?foo=1&amp;bar=2</a></p>

I also encountered this issue with myst-parser=2.0.0

$ echo '[example](https://example.com?param1=value1&param2=value2)' | myst-docutils-demo
<p><a class="reference external" href="https://example.com?param1=value1&amp;amp;param2=value2">example</a></p>

Ended up working around it using explicit HTML, e.g.;

$ echo '<a href="https://example.com?param1=value1&param2=value2">example</a>' | myst-docutils-demo
<p><a href="https://example.com?param1=value1&param2=value2">example</a></p>

Not elegant, but might get you out of a pinch!