diff -urN ../htmllint.orig/tagslist.cgi ./tagslist.cgi
--- ../htmllint.orig/tagslist.cgi	Sat Sep  8 01:00:00 2012
+++ ./tagslist.cgi	Thu Sep 13 13:00:00 2012
@@ -151,13 +151,11 @@
     $CHARSET = 'ISO-2022-JP';
   }
 }
-if ($TAGSLIST) {
-  # 規則ファイルの日付を調べる
-  $RULETIME = (stat($0))[9];
-  foreach (keys %doctypes) {
-    my $t = (stat($RULEDIR.$_.'.rul'))[9];
-    $RULETIME = $t if $RULETIME < $t;
-  }
+# 規則ファイルの日付を調べる
+$RULETIME = (stat($0))[9];
+foreach (keys %doctypes, 'common') {
+  my $t = (stat($RULEDIR.$_.'.rul'))[9];
+  $RULETIME = $t if $RULETIME < $t;
 }
 $rule = 'all' unless $rule;
 $| = 1;
@@ -166,7 +164,18 @@
             '|NUMBERS|NAMES|NMTOKENS|NUTOKENS|ID|IDREF|ENTITY';
 $internalElem = '#\d+';
 
-if ($tag) {
+if (!$nocgi &&
+    defined($ifModifiedSince = &str2time($ENV{'HTTP_IF_MODIFIED_SINCE'})) &&
+    $ifModifiedSince >= $RULETIME) {
+  # HTTPの要件(rfc2616 10.3.5)では、この場合はContent-Typeなどの
+  # エンティティヘッダを出力すべきではないとされているが、
+  # CGIの要件(rfc3875 6.2.1)では、最低でもContent-Typeは出力する必要がある。
+  print qq|Status: 304 Not Modified\x0D\x0A|,
+        qq|Content-Type: text/html; charset=$CHARSET\x0D\x0A\x0D\x0A|;
+} elsif (!$nocgi && $ENV{'REQUEST_METHOD'} eq 'HEAD') {
+  # メッセージボディを返さないことを除いてGETと同じ(rfc2616 9.4)
+  &PrintHTTPHeader;
+} elsif ($tag) {
   $tag = "\U$tag";
   &SelectTAGSLIST($tag);
   &PrintHTTPHeader unless $TAGSLIST;
@@ -561,7 +570,11 @@
 
 sub PrintHTTPHeader
 {
-  print qq|Content-Type: text/html; charset=$CHARSET\x0D\x0A\x0D\x0A| unless $nocgi;
+  unless ($nocgi) {
+    my $modtime = &time2str($RULETIME);
+    print qq|Last-Modified: $modtime\x0D\x0A|,
+          qq|Content-Type: text/html; charset=$CHARSET\x0D\x0A\x0D\x0A|;
+  }
 }
 sub PrintHTMLHeader
 {
@@ -606,6 +619,40 @@
 }
 
 ##############################################################
+# HTTP::Date互換日付変換ルーチン
+
+sub time2str
+{
+  my $time = shift;
+# $time = time unless defined($time);
+  sprintf('%s, %02d %s %d %s GMT', @{[split(/\s+/, gmtime($time))]}[0,2,1,4,3]);
+}
+
+sub str2time
+{
+  my $str = shift;
+  return undef unless defined($str);
+  my $MoY = 'Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec';
+  my @tm;
+
+  # HTTP::Dateとは異なり、後ろの余計な文字は無視する
+  if ($str =~ /^\s*[SMTWF][a-z]{2,}, +(\d\d?) +($MoY) +(\d{4}) +(\d\d):(\d\d):(\d\d) +GMT/) {
+    @tm = ($6, $5, $4, $1, index($MoY,$2)/4, $3);   # rfc1123 HTTP format
+  } elsif ($str =~ /^\s*[SMTWF][a-z]{2,}, +(\d\d?)-($MoY)-(\d{2,4}) +(\d\d):(\d\d):(\d\d) +GMT/) {
+    @tm = ($6, $5, $4, $1, index($MoY,$2)/4, $3);   # rfc850 HTTP format
+  } elsif ($str =~ /^\s*[SMTWF][a-z]{2,} +($MoY) +(\d\d?) +(\d\d):(\d\d):(\d\d) +(\d{4})/) {
+    @tm = ($5, $4, $3, $2, index($MoY,$1)/4, $6);   # ANSI C asctime() format
+  } else {
+    return undef;
+  }
+  return eval {
+    require Time::Local;
+    my $t = Time::Local::timegm(@tm);
+    $t < 0 ? undef : $t;
+  };
+}
+
+##############################################################
 
 sub SelectTAGSLIST
 {