bug#44874: 27.1; nXML: error when setting the xhtml5.rnc schema

[Vincent Lefevre]

Create a file foo.xml containing: <root></root>

Then type "emacs -Q foo.xml", and in the XML menu,
Set Schema → File..., and select xhtml5.rnc, which
can be obtained here:

  https://github.com/emacsmirror/html5-schema

(described as "Add HTML5 schemas for use by nXML").
I get the following error:

Incorrect schema. Unknown datatype library http://whattf.org/datatype-draft


In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.23, cairo 
version 1.16.0)
 of 2020-11-08, modified by Debian built on x86-ubc-01
Windowing system distributor 'The X.Org Foundation', version 11.0.12009000
System Description: Debian GNU/Linux bullseye/sid

Recent messages:
Loading /etc/emacs/site-start.d/50latex-cjk-common.el (source)...done
Loading /etc/emacs/site-start.d/50latex-cjk-thai.el (source)...done
Loading /etc/emacs/site-start.d/50latexmk.el (source)...done
Loading /etc/emacs/site-start.d/50maxima-emacs.el (source)...done
Loading /etc/emacs/site-start.d/50psvn.el (source)...done
Loading /etc/emacs/site-start.d/50texlive-lang-english.el (source)...done
Loading /etc/emacs/site-start.d/50why3.el (source)...done
Loading /home/vinc17/share/emacs/site-lisp/mutteditor.el (source)...done
Loading time...done
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --enable-libsystemd --with-pop=yes
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/27.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/27.1/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --build
 x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
 --libexecdir=/usr/lib --localstatedir=/var/lib
 --infodir=/usr/share/info --mandir=/usr/share/man --enable-libsystemd
 --with-pop=yes
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/27.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/27.1/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --with-cairo
 --with-x=yes --with-x-toolkit=gtk3 --with-toolkit-scroll-bars
 'CFLAGS=-g -O2 -fdebug-prefix-map=/build/emacs-6jKC2B/emacs-27.1+1=.
 -fstack-protector-strong -Wformat -Werror=format-security -Wall'
 'CPPFLAGS=-Wdate-time -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF
ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD
JSON PDUMPER LCMS2 GMP

Important settings:
  value of $LC_COLLATE: POSIX
  value of $LC_CTYPE: C.UTF-8
  value of $LC_TIME: en_DK.utf8
  value of $LANG: POSIX
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  display-time-mode: t
  show-paren-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
/usr/share/emacs/site-lisp/llvm-10/tablegen-mode hides 
/usr/share/emacs/site-lisp/llvm-3.5/tablegen-mode
/usr/share/emacs/site-lisp/llvm-10/llvm-mode hides 
/usr/share/emacs/site-lisp/llvm-3.5/llvm-mode
/usr/share/emacs/site-lisp/llvm-10/emacs hides 
/usr/share/emacs/site-lisp/llvm-3.5/emacs
/usr/share/emacs/site-lisp/llvm-10/tablegen-mode hides 
/usr/share/emacs/site-lisp/llvm-3.6/tablegen-mode
/usr/share/emacs/site-lisp/llvm-10/llvm-mode hides 
/usr/share/emacs/site-lisp/llvm-3.6/llvm-mode
/usr/share/emacs/site-lisp/llvm-10/emacs hides 
/usr/share/emacs/site-lisp/llvm-3.6/emacs
/usr/share/emacs/site-lisp/llvm-10/tablegen-mode hides 
/usr/share/emacs/site-lisp/llvm-3.7/tablegen-mode
/usr/share/emacs/site-lisp/llvm-10/llvm-mode hides 
/usr/share/emacs/site-lisp/llvm-3.7/llvm-mode
/usr/share/emacs/site-lisp/llvm-10/emacs hides 
/usr/share/emacs/site-lisp/llvm-3.7/emacs
/usr/share/emacs/site-lisp/llvm-10/tablegen-mode hides 
/usr/share/emacs/site-lisp/llvm-7/tablegen-mode
/usr/share/emacs/site-lisp/llvm-10/llvm-mode hides 
/usr/share/emacs/site-lisp/llvm-7/llvm-mode
/usr/share/emacs/site-lisp/llvm-10/emacs hides 
/usr/share/emacs/site-lisp/llvm-7/emacs
/usr/share/emacs/site-lisp/llvm-10/tablegen-mode hides 
/usr/share/emacs/site-lisp/llvm-8/tablegen-mode
/usr/share/emacs/site-lisp/llvm-10/llvm-mode hides 
/usr/share/emacs/site-lisp/llvm-8/llvm-mode
/usr/share/emacs/site-lisp/llvm-10/emacs hides 
/usr/share/emacs/site-lisp/llvm-8/emacs
/usr/share/emacs/site-lisp/llvm-10/tablegen-mode hides 
/usr/share/emacs/site-lisp/llvm-9/tablegen-mode
/usr/share/emacs/site-lisp/llvm-10/llvm-mode hides 
/usr/share/emacs/site-lisp/llvm-9/llvm-mode
/usr/share/emacs/site-lisp/llvm-10/emacs hides 
/usr/share/emacs/site-lisp/llvm-9/emacs
/usr/share/emacs/site-lisp/latex-cjk-thai/thai-word hides 
/usr/share/emacs/27.1/lisp/language/thai-word

Features:
(shadow sort mail-extr warnings emacsbug message rmc puny dired
dired-loaddefs format-spec rfc822 mml easymenu mml-sec password-cache
epa derived epg epg-config gnus-util rmail rmail-loaddefs
text-property-search time-date subr-x seq byte-opt gv bytecomp
byte-compile cconv mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils time cus-start cus-load paren cc-styles
cc-align cc-engine cc-vars cc-defs edmacro kmacro cl-loaddefs cl-lib
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode elisp-mode
lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow isearch
timer select scroll-bar mouse jit-lock font-lock syntax facemenu
font-core term/tty-colors frame minibuffer cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote threads dbusbind
inotify lcms2 dynamic-setting system-font-setting font-render-setting
cairo move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 68727 6543)
 (symbols 48 8993 1)
 (strings 32 22350 2450)
 (string-bytes 1 715680)
 (vectors 16 8360)
 (vector-slots 8 108969 6652)
 (floats 8 26 28)
 (intervals 56 260 0)
 (buffers 1000 12))