Go version

1.22.5 (this is from pkg.go.dev)

Output of go env in your module/workspace:

Whatever pkg.go.dev uses when this issue is filed

What did you do?

https://go.dev/play/p/rzV-FdleVIV

package main

import "encoding/xml"

func main() {
	err := xml.Unmarshal([]byte(`<?a /b?><a/>`), new(interface{}))
	if err != nil {
		panic("well-formed XML rejected")
	}
	err = xml.Unmarshal([]byte(`<?a/b?><a/>`), new(interface{}))
	if err == nil {
		panic("missing space before processing instruction value accepted")
	}
}

What did you see happen?

<?a/b?><a/> is parsed successfully.

What did you expect to see?

Error because <?a/b?> is not a well-formed processing instruction. There must be whitespace between the processing instruction target and value.