diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -5771,6 +5771,7 @@
     'PhutilRemarkupQuotesBlockRule' => 'infrastructure/markup/blockrule/PhutilRemarkupQuotesBlockRule.php',
     'PhutilRemarkupReplyBlockRule' => 'infrastructure/markup/blockrule/PhutilRemarkupReplyBlockRule.php',
     'PhutilRemarkupRule' => 'infrastructure/markup/markuprule/PhutilRemarkupRule.php',
+    'PhutilRemarkupRuleTestCase' => 'infrastructure/markup/markuprule/__tests__/PhutilRemarkupRuleTestCase.php',
     'PhutilRemarkupSimpleTableBlockRule' => 'infrastructure/markup/blockrule/PhutilRemarkupSimpleTableBlockRule.php',
     'PhutilRemarkupTableBlockRule' => 'infrastructure/markup/blockrule/PhutilRemarkupTableBlockRule.php',
     'PhutilRemarkupTestInterpreterRule' => 'infrastructure/markup/blockrule/PhutilRemarkupTestInterpreterRule.php',
@@ -12668,6 +12669,7 @@
     'PhutilRemarkupQuotesBlockRule' => 'PhutilRemarkupQuotedBlockRule',
     'PhutilRemarkupReplyBlockRule' => 'PhutilRemarkupQuotedBlockRule',
     'PhutilRemarkupRule' => 'Phobject',
+    'PhutilRemarkupRuleTestCase' => 'PhabricatorTestCase',
     'PhutilRemarkupSimpleTableBlockRule' => 'PhutilRemarkupBlockRule',
     'PhutilRemarkupTableBlockRule' => 'PhutilRemarkupBlockRule',
     'PhutilRemarkupTestInterpreterRule' => 'PhutilRemarkupBlockInterpreter',
diff --git a/src/infrastructure/markup/markuprule/PhutilRemarkupDocumentLinkRule.php b/src/infrastructure/markup/markuprule/PhutilRemarkupDocumentLinkRule.php
--- a/src/infrastructure/markup/markuprule/PhutilRemarkupDocumentLinkRule.php
+++ b/src/infrastructure/markup/markuprule/PhutilRemarkupDocumentLinkRule.php
@@ -45,11 +45,11 @@
     $engine = $this->getEngine();
 
     $is_anchor = false;
-    if (strncmp($link, '/', 1) == 0) {
+    if (self::isURIRelativePath($link)) {
       $base = phutil_string_cast($engine->getConfig('uri.base'));
       $base = rtrim($base, '/');
       $link = $base.$link;
-    } else if (strncmp($link, '#', 1) == 0) {
+    } else if (self::isURIAnchor($link)) {
       $here = $engine->getConfig('uri.here');
       $link = $here.$link;
 
@@ -76,7 +76,8 @@
       return $name;
     }
 
-    $same_window = $engine->getConfig('uri.same-window', false);
+    $is_internal = self::isURIInternal($link);
+    $same_window = $engine->getConfig('uri.same-window', $is_internal);
     if ($same_window) {
       $target = null;
     } else {
@@ -88,11 +89,18 @@
       $target = null;
     }
 
+    // Allow developers to style esternal links differently
+    $classes = array('remarkup-link');
+    if (!$is_internal) {
+      $classes[] = 'remarkup-link-ext';
+    }
+    $classes = implode(' ', $classes);
+
     return phutil_tag(
       'a',
       array(
         'href' => $link,
-        'class' => 'remarkup-link',
+        'class' => $classes,
         'target' => $target,
         'rel' => 'noreferrer',
       ),
diff --git a/src/infrastructure/markup/markuprule/PhutilRemarkupHyperlinkRule.php b/src/infrastructure/markup/markuprule/PhutilRemarkupHyperlinkRule.php
--- a/src/infrastructure/markup/markuprule/PhutilRemarkupHyperlinkRule.php
+++ b/src/infrastructure/markup/markuprule/PhutilRemarkupHyperlinkRule.php
@@ -116,18 +116,26 @@
 
     $engine = $this->getEngine();
 
-    $same_window = $engine->getConfig('uri.same-window', false);
+    $is_internal = self::isURIAbsoluteInternal($link);
+    $same_window = $engine->getConfig('uri.same-window', $is_internal);
     if ($same_window) {
       $target = null;
     } else {
       $target = '_blank';
     }
 
+    // Allow web designers to style external links differently
+    $classes = array('remarkup-link');
+    if (!$is_internal) {
+      $classes[] = 'remarkup-link-ext';
+    }
+    $classes = implode(' ', $classes);
+
     return phutil_tag(
       'a',
       array(
         'href' => $link,
-        'class' => 'remarkup-link',
+        'class' => $classes,
         'target' => $target,
         'rel' => 'noreferrer',
       ),
diff --git a/src/infrastructure/markup/markuprule/PhutilRemarkupRule.php b/src/infrastructure/markup/markuprule/PhutilRemarkupRule.php
--- a/src/infrastructure/markup/markuprule/PhutilRemarkupRule.php
+++ b/src/infrastructure/markup/markuprule/PhutilRemarkupRule.php
@@ -106,4 +106,73 @@
     return (strpos($text, PhutilRemarkupBlockStorage::MAGIC_BYTE) === false);
   }
 
+  /**
+   * Check if a generic URI points to an internal destination
+   *
+   * The URI can be relative or absolute
+   *
+   * WARNING: This method is public just to make unit tests, so,
+   * don't adopt this method in production externally since it can
+   * change anytime.
+   * If you need to check this from external places, please
+   * propose to move the business logic somewhere else, but please
+   * also indicating where.
+   * https://we.phorge.it/T15182
+   *
+   * @param string $uri Example '/bar/' or 'http://example.com/bar/'
+   * @return bool
+   */
+  public static function isURIInternal($uri) {
+    return self::isURIAnchor($uri)
+        || self::isURIRelativePath($uri)
+        || self::isURIAbsoluteInternal($uri);
+  }
+
+  /**
+   * Check whenever an URI is an anchor.
+   *
+   * @param string $uri
+   * @return bool
+   */
+  protected static function isURIAnchor($uri) {
+    return strncmp($uri, '#', 1) === 0;
+  }
+
+  /**
+   * Check whenever an URI is a relative one.
+   *
+   * @param string $uri Example '/path' etc.
+   * @return bool
+   */
+  protected static function isURIRelativePath($uri) {
+    // is this starting with '/' or './' or '../'?
+    return
+      strncmp($uri, '/',   1) === 0 ||
+      strncmp($uri, './',  2) === 0 ||
+      strncmp($uri, '../', 3) === 0;
+  }
+
+  /**
+   * Check if an absolute URI points to Phorge/Phabricator itself.
+   *
+   * @param string $uri Example 'http://example.com/bar/'
+   * @return bool
+   */
+  protected static function isURIAbsoluteInternal($uri) {
+
+    // Get the Base URI - full URI with protocol and ending with '/'
+    // It's difficult to imagine a case where this config is not defined,
+    // but better to be nice here.
+    $base_uri =
+      PhabricatorEnv::getEnvConfigIfExists('phabricator.base-uri', null);
+
+    // Assume a safe default.
+    if (!$base_uri) {
+      return false;
+    }
+
+    // Check if the URI starts with the base URI.
+    return strncmp($uri, $base_uri, strlen($base_uri)) === 0;
+  }
+
 }
diff --git a/src/infrastructure/markup/markuprule/__tests__/PhutilRemarkupRuleTestCase.php b/src/infrastructure/markup/markuprule/__tests__/PhutilRemarkupRuleTestCase.php
new file mode 100644
--- /dev/null
+++ b/src/infrastructure/markup/markuprule/__tests__/PhutilRemarkupRuleTestCase.php
@@ -0,0 +1,34 @@
+<?php
+
+final class PhutilRemarkupRuleTestCase extends PhabricatorTestCase {
+
+  public function testIsURIInternal() {
+    $tests = array(
+      // name, value, expected
+      array('anchor', '#asd', true),
+      array('relative dir', '/foo/', true),
+      array('root dir', '/', true),
+      array('external uri', 'https://gnu.org/', false),
+    );
+
+    // if the base URI is set, also try against ourself
+    $base_uri =
+      PhabricatorEnv::getEnvConfigIfExists('phabricator.base-uri', null);
+    if ($base_uri) {
+      // name, value, expected
+      $tests[] = array('base uri', $base_uri, true);
+    }
+
+    foreach ($tests as $test) {
+      $test_name = $test[0];
+      $test_value = $test[1];
+      $test_expected = $test[2];
+
+      // This method is just exposed to do unit tests here.
+      // Our goal is not to consider this method as stable.
+      $tested_value = PhutilRemarkupRule::isURIInternal($test_value);
+
+      $this->assertEqual($tested_value, $test_expected, $test_name);
+    }
+  }
+}