I thought it was intentional and not a bug - thanks for reporting! It's true that I don't see flash-of-unstyled-content on HTML and XHTML pages, unless the CSS file is delayed by many seconds. I noticed another bug though - the Firefox inspector tool doesn't show style rules for the XML document.
https://bugzilla.mozilla.org/show_bug.cgi?id=1506038
This bug is actually a regression in Firefox 58 from the fix for a use-after-free bug (CVE-2018-5097):
https://bugzilla.mozilla.org/show_bug.cgi?id=1387427