bug#51525: 27.2; shr.el fails to render <abbr> tags missing a title attribute

[Greg Farough]

It looks like shr.el will skip over rendering HTML <abbr> tags that
are missing the "title" attribute. It seems a little strange to me,
but according to MDN[1] that attribute is optional, and examples of it
being omitted can be found out in the wild.

To verify this, you can use eww.el to open this test page[2] I've
written on Emacs 27.2 or the master branch. The first paragraph
contains an <abbr> tag with a title attribute, and the second one
contains an <abbr> tag that does not.

As a bonus, I have a real world and holiday-appropriate example[3] from
Bram Stoker's Dracula. Search for "welcome," and you should see this:

"I am Dracula; and I bid you welcome,  Harker, to my house."

The extra space before Harker is due to "Mr." being enclosed in an
<abbr> tag that has no title attribute.

Not a major bug by any means, but I thought I'd let you all know. Thanks
very much.

-g

[1]: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/abbr
[2]: https://gregf.beevomit.org/abbr.html
[3]: https://standardebooks.org/ebooks/bram-stoker/dracula/text/chapter-2

In GNU Emacs 27.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo 
version 1.16.0)
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
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CONFIG_SHELL=/gnu/store/pwcp239kjf7lnj5i4lkdzcfcxwcfyk72-bash-minimal-5.0.16/bin/bash
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Configured features:
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INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF
ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS JSON PDUMPER
GMP

Important settings:
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/gnu/store/vdxz1c1s2cmy4qynj8vmz8nvpwvl31w0-profile/share/emacs/site-lisp:/gnu/store/ymvcv4shwva26r3k9mxh5d38hsbrygpb-emacs-27.2/share/emacs/27.2/lisp
  locale-coding-system: nil

Major mode: Lisp Interaction

Minor modes in effect:
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  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
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syntax facemenu font-core term/tty-colors frame minibuffer cl-generic
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custom widget hashtable-print-readable backquote threads dbusbind
inotify dynamic-setting system-font-setting font-render-setting cairo
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Memory information:
((conses 16 43402 10579)
 (symbols 48 6002 1)
 (strings 32 15181 1838)
 (string-bytes 1 505260)
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 (vector-slots 8 75937 8440)
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 (buffers 1000 11))

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this minute."