* lisp/progmodes/js.el (js-mode): Use "\\(?:" in the value of
comment-start-skip rather than "\\(", fixing the second half of bug #41952.
Call c-foreign-init-lit-pos-cache and install c-foreign-truncate-lit-pos-cache
on before-change-functions, to connect up correctly with CC Mode's filling
mechanism.

* lisp/textmodes/mhtml-mode.el (mhtml--crucial-variable-prefix): Add prefixes
"adaptive-fill-", "fill-", "normal-auto-fill-function" and "paragraph-" to
pull in variables crucial to filling.
(mhtml-syntax-propertize): Read the current submode from the piece of text
being propertized rather than one character before it, and do so before
erasing the submode text-property.
(mhtml-mode): Set the js-mode value of auto-fill-function to js-do-auto-fill.
Correctly initialize and use CC Mode's filling facilities, as above.

* lisp/cedet/semantic/complete.el
(semantic-displayer-tooltip-max-tags):
* lisp/emacs-lisp/benchmark.el (benchmark-run-compiled):
* lisp/emacs-lisp/package.el (package--default-summary)
(package-menu-filter-by-version):
* lisp/eshell/em-pred.el (eshell-pred-file-time):
* lisp/progmodes/verilog-mode.el (verilog-auto-lineup)
(verilog-auto-reset-widths, verilog-auto-arg-format)
(verilog-auto-inst-vector, verilog-auto-inst-template-numbers):
* lisp/textmodes/bibtex.el (bibtex-dialect):
* test/lisp/autoinsert-tests.el
(autoinsert-tests-define-auto-insert-before)
(autoinsert-tests-define-auto-insert-after): Remove some unnecessary
quoting around anonymous functions.

* lisp/format-spec.el: Use lexical-binding.  Remove dependence on
subr-x.el.
(format-spec-make): Clarify docstring.
(format-spec--parse-modifiers): Rename to...
(format-spec--parse-flags): ...this and simplify.  In particular,
don't bother parsing :space-pad which is redundant and unused.
(format-spec--pad): Remove, replacing with...
(format-spec--do-flags): ...this new helper function which performs
more of format-spec's supported text manipulation.
(format-spec): Autoload.  Allow optional argument to take on special
values 'ignore' and 'delete' for more control over what happens when
a replacement for a format specification isn't provided.  Bring back
proper support for a precision modifier similar to that of 'format'.

* lisp/battery.el (battery-format): Rewrite in terms of format-spec.
(battery-echo-area-format, battery-mode-line-format): Mention
support of format-spec syntax in docstrings.

* doc/lispref/strings.texi (Custom Format Strings):
* etc/NEWS: Document and announce these changes.

* lisp/dired-aux.el (dired-do-compress-to):
* lisp/erc/erc-match.el (erc-log-matches):
* lisp/erc/erc.el (erc-update-mode-line-buffer):
* lisp/gnus/gnus-sieve.el (gnus-sieve-update):
* lisp/gnus/gssapi.el (open-gssapi-stream):
* lisp/gnus/mail-source.el (mail-source-fetch-file)
(mail-source-fetch-directory, mail-source-fetch-pop)
(mail-source-fetch-imap):
* lisp/gnus/message.el (message-insert-formatted-citation-line):
* lisp/image-dired.el:
* lisp/net/eww.el:
* lisp/net/imap.el (imap-kerberos4-open, imap-gssapi-open)
(imap-shell-open):
* lisp/net/network-stream.el (network-stream-open-shell):
* lisp/obsolete/tls.el (open-tls-stream):
* lisp/textmodes/tex-mode.el:
Remove extraneous loads and autoloads of format-spec now that it is
autoloaded and simplify its uses where possible.

* test/lisp/battery-tests.el (battery-format): Test new format-spec
support.
* test/lisp/format-spec-tests.el (test-format-spec): Rename to...
(format-spec) ...this, extending test cases.
(test-format-unknown): Rename to...
(format-spec-unknown): ...this, extending test cases.
(test-format-modifiers): Rename to...
(format-spec-flags): ...this.
(format-spec-make, format-spec-parse-flags, format-spec-do-flags)
(format-spec-do-flags-truncate, format-spec-do-flags-pad)
(format-spec-do-flags-chop, format-spec-do-flags-case): New tests.

Add a predicate, color-dark-p, for deciding whether a colour is more
readable with black or white as contrast.  It has experimentally been
shown to be more accurate and robust than the various methods
currently employed.

The new predicate compares the relative luminance of the colour to an
empirically determined cut-off value, and it seems to get it right in
almost all cases, with no value leading to outright bad results.

* lisp/faces.el (readable-foreground-color): Use color-dark-p.
(color-dark-p): New function.
* lisp/facemenu.el (list-colors-print): Use readable-foreground-color,
improving readability of list-colors-display.
* lisp/textmodes/css-mode.el (css--contrasty-color): Remove.
(css--fontify-region): Use readable-foreground-color.

* lisp/textmodes/mhtml-mode.el (mhtml--extend-font-lock-region):
Remove.
(mhtml-mode): Don't set font-lock-extend-region-functions.

* lisp/textmodes/bibtex.el
(bibtex-autokey-before-presentation-function): Bump :version tag now
that nil is no longer a valid value.

In the top-level construction of the entry-type commands,
use `defalias` instead of (eval `(defun ...)).

(bibtex-insert-kill): Strength reduce `eval` => `symbol-value`.
(bibtex-autokey-before-presentation-function): Avoid nil value.

* lisp/textmodes/css-mode.el (css-nmchar-re): Allow underscores in
variable names (and in identifiers in general).

* test/manual/indent/css-mode.css: Add some examples of variable names
with underscores in them.

* test/manual/indent/less-css-mode.less: Add some examples of variable
names with underscores in them.

* test/manual/indent/scss-mode.scss: Add some examples of variable
names with underscores in them.

* lisp/textmodes/css-mode.el (css-property-alist): Add new properties
from CSS Containment Module Level 1.