––> Details, demo code, screenshots <––

This bug took me quite a while to understand, because when using the file:// protocol (which I did), a decoy non-error is reported at the same time as the real error is silenced:

XML Parsing Error: not well-formed Location: file:///mnt/…/curl-cjsm11-bug/01-totally-common.js Line Number 1, Column 13