Radical cleanup of COMPATIBILITY sections:

Remove lots of lies, dozens of irrelevant implementation details,
and all references to groff versions older than 1.17.  Move relevant
information to the pages where it belongs, and out of mandoc(1) in
particular.  Add some missing general remarks to roff(7), where it
fits the character and purpose of the page much better.

6 files changed, 80 insertions, 299 deletions

diff --git a/eqn.7 b/eqn.7
index 045c0a55..151555ba 100644
--- a/eqn.7
+++ b/eqn.7
@@ -457,11 +457,6 @@ The
 and
 .Cm down Ar n
 commands are also ignored.
-.It
-Inline equations and the
-.Cm delim
-control statement are not yet implemented in
-.Xr mandoc 1 .
 .El
 .Sh SEE ALSO
 .Xr mandoc 1 ,
diff --git a/man.7 b/man.7
index 7311e707..9fda75b6 100644
--- a/man.7
+++ b/man.7
@@ -894,53 +894,6 @@ until the end of the macro scope.
 Note that macros like
 .Sx \&BR
 open and close a font scope for each argument.
-.Sh COMPATIBILITY
-This section mentions some areas of questionable portability between
-implementations of the
-.Nm
-language.
-More incompatibilities exist.
-.Pp
-.Bl -dash -compact
-.It
-Do not depend on
-.Sx \&SH
-or
-.Sx \&SS
-to close out a literal context opened with
-.Sx \&nf .
-This behaviour may not be portable.
-.It
-troff suppresses a newline before
-.Sq \(aq
-macro output; in mandoc, it is an alias for the standard
-.Sq \&.
-control character.
-.It
-In page header lines, GNU troff versions up to and including 1.21
-only print
-.Ar volume
-names explicitly specified in the
-.Sx \&TH
-macro; mandoc and newer groff print the default volume name
-corresponding to the
-.Ar section
-number when no
-.Ar volume
-is given, like in
-.Xr mdoc 7 .
-.El
-.Pp
-The
-.Sx EE ,
-.Sx EX ,
-.Sx OP ,
-.Sx UE ,
-and
-.Sx UR
-macros are part of the GNU extended
-.Nm
-macro set, and may not be portable to non-GNU troff implementations.
 .Sh SEE ALSO
 .Xr man 1 ,
 .Xr mandoc 1 ,
diff --git a/mandoc.1 b/mandoc.1
index cef67da2..275e35dd 100644
--- a/mandoc.1
+++ b/mandoc.1
@@ -321,9 +321,6 @@ Emboldened characters are rendered as
 The special characters documented in
 .Xr mandoc_char 7
 are rendered best-effort in an ASCII equivalent.
-If no equivalent is found,
-.Sq \&?
-is used instead.
 .Pp
 Output width is limited to 78 visible columns unless literal input lines
 exceed this limit.
@@ -344,7 +341,7 @@ for example overfull lines or ugly line breaks.
 .It Cm width Ns = Ns Ar width
 The output width is set to
 .Ar width ,
-which will normalise to \(>=60.
+which will normalise to \(>=58.
 .El
 .Ss HTML Output
 Output produced by
@@ -1670,88 +1667,9 @@ macro or of an undefined macro.
 The macro is ignored, and its arguments are handled
 as if they were a text line.
 .El
-.Sh COMPATIBILITY
-This section summarises
-.Nm
-compatibility with GNU troff.
-Each input and output format is separately noted.
-.Ss ASCII Compatibility
-.Bl -bullet -compact
-.It
-Unrenderable unicode codepoints specified with
-.Sq \e[uNNNN]
-escapes are printed as
-.Sq \&?
-in mandoc.
-In GNU troff, these raise an error.
-.It
-The
-.Sq \&Bd \-literal
-and
-.Sq \&Bd \-unfilled
-macros of
-.Xr mdoc 7
-in
-.Fl T Ns Cm ascii
-are synonyms, as are \-filled and \-ragged.
-.It
-In historic GNU troff, the
-.Sq \&Pa
-.Xr mdoc 7
-macro does not underline when scoped under an
-.Sq \&It
-in the FILES section.
-This behaves correctly in
-.Nm .
-.It
-A list or display following the
-.Sq \&Ss
-.Xr mdoc 7
-macro in
-.Fl T Ns Cm ascii
-does not assert a prior vertical break, just as it doesn't with
-.Sq \&Sh .
-.It
-The
-.Sq \&na
-.Xr man 7
-macro in
-.Fl T Ns Cm ascii
-has no effect.
-.It
-Words aren't hyphenated.
-.El
-.Ss HTML Compatibility
-.Bl -bullet -compact
-.It
-The
-.Sq \efP
-escape will revert the font to the previous
-.Sq \ef
-escape, not to the last rendered decoration, which is now dictated by
-CSS instead of hard-coded.
-It also will not span past the current scope,
-for the same reason.
-Note that in
-.Sx ASCII Output
-mode, this will work fine.
-.It
-The
-.Xr mdoc 7
-.Sq \&Bl \-hang
-and
-.Sq \&Bl \-tag
-list types render similarly (no break following overreached left-hand
-side) due to the expressive constraints of HTML.
-.It
-The
-.Xr man 7
-.Sq IP
-and
-.Sq TP
-lists render similarly.
-.El
 .Sh SEE ALSO
+.Xr apropos 1 ,
+.Xr man 1 ,
 .Xr eqn 7 ,
 .Xr man 7 ,
 .Xr mandoc_char 7 ,
@@ -1762,32 +1680,15 @@ lists render similarly.
 The
 .Nm
 utility was written by
-.An Kristaps Dzonsons Aq Mt kristaps@bsd.lv .
-.Sh CAVEATS
+.An Kristaps Dzonsons Aq Mt kristaps@bsd.lv
+and is maintained by
+.An Ingo Schwarze Aq Mt schwarze@openbsd.org .
+.Sh BUGS
 In
-.Fl T Ns Cm html
-and
-.Fl T Ns Cm xhtml ,
+.Fl T Ns Cm html ,
 the maximum size of an element attribute is determined by
 .Dv BUFSIZ ,
 which is usually 1024 bytes.
 Be aware of this when setting long link
 formats such as
 .Fl O Ns Cm style Ns = Ns Ar really/long/link .
-.Pp
-Nesting elements within next-line element scopes of
-.Fl m Ns Cm an ,
-such as
-.Sq br
-within an empty
-.Sq B ,
-will confuse
-.Fl T Ns Cm html
-and
-.Fl T Ns Cm xhtml
-and cause them to forget the formatting of the prior next-line scope.
-.Pp
-The
-.Sq \(aq
-control character is an alias for the standard macro control character
-and does not emit a line-break as stipulated in GNU troff.
diff --git a/mdoc.7 b/mdoc.7
index 05f13e24..234364c9 100644
--- a/mdoc.7
+++ b/mdoc.7
@@ -3133,50 +3133,13 @@ Manually switching the font using the
 font escape sequences is never required.
 .Sh COMPATIBILITY
 This section provides an incomplete list of compatibility issues
-between mandoc and other troff implementations, at this time limited
-to GNU troff
+between mandoc and GNU troff
 .Pq Qq groff .
-The term
-.Qq historic groff
-refers to groff versions before 1.17,
-which featured a significant update of the
-.Pa doc.tmac
-file.
-.Pp
-Heirloom troff, the other significant troff implementation accepting
-\-mdoc, is similar to historic groff.
 .Pp
 The following problematic behaviour is found in groff:
-.ds hist (Historic groff only.)
 .Pp
 .Bl -dash -compact
 .It
-Display macros
-.Po
-.Sx \&Bd ,
-.Sx \&Dl ,
-and
-.Sx \&D1
-.Pc
-may not be nested.
-\*[hist]
-.It
-.Sx \&At
-with unknown arguments produces no output at all.
-\*[hist]
-Newer groff and mandoc print
-.Qq AT&T UNIX
-and the arguments.
-.It
-.Sx \&Bl Fl column
-does not recognise trailing punctuation characters when they immediately
-precede tabulator characters, but treats them as normal text and
-outputs a space before them.
-.It
-.Sx \&Bd Fl ragged compact
-does not start a new line.
-\*[hist]
-.It
 .Sx \&Dd
 with non-standard arguments behaves very strangely.
 When there are three arguments, they are printed verbatim.
@@ -3185,53 +3148,6 @@ but without any arguments the string
 .Dq Epoch
 is printed.
 .It
-.Sx \&Fl
-does not print a dash for an empty argument.
-\*[hist]
-.It
-.Sx \&Fn
-does not start a new line unless invoked as the line macro in the
-.Em SYNOPSIS
-section.
-\*[hist]
-.It
-.Sx \&Fo
-with
-.Pf non- Sx \&Fa
-children causes inconsistent spacing between arguments.
-In mandoc, a single space is always inserted between arguments.
-.It
-.Sx \&Ft
-in the
-.Em SYNOPSIS
-causes inconsistent vertical spacing, depending on whether a prior
-.Sx \&Fn
-has been invoked.
-See
-.Sx \&Ft
-and
-.Sx \&Fn
-for the normalised behaviour in mandoc.
-.It
-.Sx \&In
-ignores additional arguments and is not treated specially in the
-.Em SYNOPSIS .
-\*[hist]
-.It
-.Sx \&It
-sometimes requires a
-.Fl nested
-flag.
-\*[hist]
-In new groff and mandoc, any list may be nested by default and
-.Fl enum
-lists will restart the sequence only for the sub-list.
-.It
-.Sx \&Li
-followed by a delimiter is incorrectly used in some manuals
-instead of properly quoting that character, which sometimes works with
-historic groff.
-.It
 .Sx \&Lk
 only accepts a single link-name argument; the remainder is misformatted.
 .It
@@ -3245,19 +3161,6 @@ can only be called by other macros, but not at the beginning of a line.
 .Sx \&%C
 is not implemented (up to and including groff-1.22.2).
 .It
-Historic groff only allows up to eight or nine arguments per macro input
-line, depending on the exact situation.
-Providing more arguments causes garbled output.
-The number of arguments on one input line is not limited with mandoc.
-.It
-Historic groff has many un-callable macros.
-Most of these (excluding some block-level macros) are callable
-in new groff and mandoc.
-.It
-.Sq \(ba
-(vertical bar) is not fully supported as a delimiter.
-\*[hist]
-.It
 .Sq \ef
 .Pq font face
 and
@@ -3275,12 +3178,26 @@ The following features are unimplemented in mandoc:
 .Bl -dash -compact
 .It
 .Sx \&Bd
-.Fl file Ar file .
+.Fl file Ar file
+is unsupported for security reasons.
+.It
+.Sx \&Bd
+.Fl filled
+does not adjust the right margin, but is an alias for
+.Sx \&Bd
+.Fl ragged .
+.It
+.Sx \&Bd
+.Fl literal
+does not use a literal font, but is an alias for
+.Sx \&Bd
+.Fl unfilled .
 .It
 .Sx \&Bd
 .Fl offset Cm center
 and
-.Fl offset Cm right .
+.Fl offset Cm right
+don't work.
 Groff does not implement centered and flush-right rendering either,
 but produces large indentations.
 .El
diff --git a/roff.7 b/roff.7
index efe53e74..81413919 100644
--- a/roff.7
+++ b/roff.7
@@ -2028,50 +2028,66 @@ approximated in
 .Xr mandoc 1
 by simply skipping the next character.
 .Sh COMPATIBILITY
-This section documents compatibility between mandoc and other
+The
+.Xr mandoc 1
+implementation of the
 .Nm
-implementations, at this time limited to GNU troff
-.Pq Qq groff .
-The term
-.Qq historic groff
-refers to groff version 1.15.
+language is intentionally incomplete.
+Unimplemented features include:
 .Pp
 .Bl -dash -compact
 .It
+For security reasons,
+.Xr mandoc 1
+never reads or writes external files except via
+.Sx \&so
+requests with safe relative paths.
+.It
+There is no automatic hyphenation, no adjustment to the right margin,
+and no centering; the output is always set flush-left.
+.It
+Support for setting tabulator positions
+and tabulator and leader characters is missing,
+and support for manually changing indentation is limited.
+.It
 The
 .Sq u
 scaling unit is the default terminal unit.
-In traditional troff systems, this unit would change depending on the
+In traditional troff systems, this unit changes depending on the
 output media.
 .It
-In mandoc, the
-.Sx \&EQ ,
-.Sx \&TE ,
-.Sx \&TS ,
-and
-.Sx \&T& ,
-macros are considered regular macros.
-In all other
-.Nm
-implementations, these are special macros that must be specified without
-spacing between the control character (which must be a period) and the
-macro name.
+Width measurements are implemented in a crude way
+and often yield wrong results.
+Explicit movement requests and escapes are ignored.
 .It
-The
-.Cm nS
-register is only compatible with OpenBSD's groff-1.15.
+There is no concept of output pages, no support for floats,
+graphics drawing, and picture inclusion;
+terminal output is always continuous.
 .It
-Historic groff did not accept white-space before a custom
-.Ar end
-macro for the
-.Sx \&ig
-request.
+Requests regarding color, font families, and glyph manipulation
+are ignored.
+Font support is very limited.
+Kerning is not implemented, and no ligatures are produced.
 .It
 The
-.Sx \&if
-and family would print funny white-spaces with historic groff when
-using the next-line syntax.
+.Qq \(aq
+macro control character does not suppress output line breaks.
+.It
+Diversions are not implemented,
+and support for traps is very incomplete.
+.It
+While recursion is supported,
+.Sx \&while
+loops are not.
 .El
+.Pp
+The special semantics of the
+.Cm nS
+number register is an idiosyncracy of
+.Ox
+manuals and not supported by other
+.Xr mdoc 7
+implementations.
 .Sh SEE ALSO
 .Xr mandoc 1 ,
 .Xr eqn 7 ,
diff --git a/tbl.7 b/tbl.7
index 79595f53..23bd01b3 100644
--- a/tbl.7
+++ b/tbl.7
@@ -328,18 +328,17 @@ It may then be followed by a tab
 .Pq or as designated by Cm tab
 or an end-of-line to terminate the row.
 .Sh COMPATIBILITY
-This section documents compatibility between mandoc and other
-.Nm
-implementations, at this time limited to GNU tbl.
-.Pp
-.Bl -dash -compact
-.It
-In GNU tbl, comments and macros are disallowed prior to the data block
-of a table.
 The
 .Xr mandoc 1
-implementation allows them.
-.El
+implementation of
+.Nm
+doesn't support
+.Xr mdoc 7
+and
+.Xr man 7
+macros and
+.Xr eqn 7
+equations inside tables.
 .Sh SEE ALSO
 .Xr mandoc 1 ,
 .Xr man 7 ,