Use a custom Lynx style sheet (LSS file) to work around Lynx bug

Lynx uses a very naïve method of applying styles to HTML elements by
hashing the element type and the class name to generate a map of
hash:style. After the hash is calculated, Lynx does not go back and
check whether or not the actual string values match the LSS properties.

See the following links on the Links mailing list:
 * http://lists.gnu.org/archive/html/lynx-dev/2015-12/msg00037.html
 * http://lists.gnu.org/archive/html/lynx-dev/2015-12/msg00039.html

This patch copies the default Lynx stylesheet but removes highlighting
and other styles that would result in unreadable text (due to not enough
contrast with the background color), and if the `help` builtin detects
that the best web browser to use is Lynx, it instructs it to use this
modified stylesheet.

doc_src/lynx.lss

@@ -0,0 +1,104 @@
+# Lynx Style Sheet
+# Used as a custom style sheet (LSS) for the Lynx browser to work around some
+# poor defaults. Usage: `lynx -lss=style.lss`
+#
+# The next line (beginning with "em") means: use bold if mono, otherwise
+# brightblue on <defaultbackground> (implicit)
+em:bold:brightblue
+strong:bold:brightred
+b:bold:red
+i:bold:brightblue
+a:bold:green
+img:dim:brown
+fig:normal:gray
+caption:reverse:brown
+hr:normal:yellow
+blockquote:normal:brightblue
+ul:normal:brown
+address:normal:magenta
+title:normal:magenta
+tt:dim:brightmagenta:default
+h1:bold:yellow
+h2:normal:brown
+h3:normal:green
+h4:normal:cyan
+label:normal:magenta
+q:normal:yellow
+small:dim:default
+big:bold:yellow
+sup:bold:yellow
+sub:dim:gray
+lh:bold:yellow
+li:normal:magenta
+code:normal:cyan
+cite:normal:cyan
+
+table:normal:brightcyan
+tr:bold:brown
+td:normal:default
+br:normal:default
+
+# Special styles - not corresponding directly to HTML tags
+#	alert	- status bar, when message begins "Alert".
+#	alink	- active link
+#	normal	- default attributes
+#	status	- status bar
+#	whereis	- whereis search target
+#
+#normal:normal:default:blue
+alink:reverse:yellow
+status:reverse:yellow
+alert:bold:yellow:red
+whereis:reverse+underline:magenta
+# currently not used
+#value:normal:green
+#high:bold:brightmagenta
+forwbackw.arrow:reverse
+
+# Styles with classes - <ul class=red> etc.
+ul.red:underline:brightred
+ul.blue:bold:brightblue
+li.red:reverse:red
+li.blue:bold:blue
+strong.a:bold:black
+/* em.a:reverse:black */
+em.a:bold:bold
+strong.b:bold:white
+em.b:reverse:white
+strong.debug:reverse:green
+font.letter:normal:default
+input.submit:normal:cyan
+tr.baone:bold:yellow
+tr.batwo:bold:green
+tr.bathree:bold:red
+#
+# Special handling for link.
+link:normal:white
+link.green:bold:brightgreen
+link.red:bold:black
+link.blue:bold:white
+link.toc:bold:black:white
+# Special cases for link - the rel or title is appended after the class.
+# <link rel=next class=red href="1">
+link.red.next:bold:red
+link.red.prev:bold:yellow
+link.blue.prev:bold:yellow
+link.blue.next:bold:blue
+link.green.toc:bold:white
+#
+# Define styles that will be used when syntax highlighting is requested
+# (commandline option -prettysrc).
+span.htmlsrc_comment:normal:white
+span.htmlsrc_tag:normal:white
+#If you don't like that the tag name and attribute name are displayed
+#in different colors, comment the following line.
+span.htmlsrc_attrib:normal:cyan
+span.htmlsrc_attrval:normal:magenta
+span.htmlsrc_abracket:normal:white
+span.htmlsrc_entity:normal:white
+##span.htmlsrc_href:
+##span.htmlsrc_entire:
+span.htmlsrc_badseq:normal:red
+span.htmlsrc_badtag:normal:red
+span.htmlsrc_badattr:normal:red
+span.htmlsrc_sgmlspecial:normal:yellow

share/functions/help.fish

@@ -148,6 +148,12 @@ function help --description 'Show help for the fish shell'
         end
         eval "$fish_browser $page_url &"
     else
+        # Work around lynx bug where <div class="contents"> always has the same formatting as links (unreadable)
+        # by using a custom style sheet. See https://github.com/fish-shell/fish-shell/issues/4170
+        set -l local_file 0
+        if eval $fish_browser --version 2>/dev/null | string match -qr Lynx
+            set fish_browser $fish_browser -lss={$__fish_help_dir}/fish.lss
+        end
         eval $fish_browser $page_url
     end
 end