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
@@ -5789,6 +5789,8 @@
     'PhutilTranslatedHTMLTestCase' => 'infrastructure/markup/__tests__/PhutilTranslatedHTMLTestCase.php',
     'PhutilTwitchAuthAdapter' => 'applications/auth/adapter/PhutilTwitchAuthAdapter.php',
     'PhutilTwitterAuthAdapter' => 'applications/auth/adapter/PhutilTwitterAuthAdapter.php',
+    'PhutilURIGoodie' => 'infrastructure/parser/PhutilURIGoodie.php',
+    'PhutilURIGoodieTestCase' => 'infrastructure/parser/__tests__/PhutilURIGoodieTestCase.php',
     'PhutilWordPressAuthAdapter' => 'applications/auth/adapter/PhutilWordPressAuthAdapter.php',
     'PhutilXHPASTSyntaxHighlighter' => 'infrastructure/markup/syntax/highlighter/PhutilXHPASTSyntaxHighlighter.php',
     'PhutilXHPASTSyntaxHighlighterFuture' => 'infrastructure/markup/syntax/highlighter/xhpast/PhutilXHPASTSyntaxHighlighterFuture.php',
@@ -12675,6 +12677,8 @@
     'PhutilTranslatedHTMLTestCase' => 'PhutilTestCase',
     'PhutilTwitchAuthAdapter' => 'PhutilOAuthAuthAdapter',
     'PhutilTwitterAuthAdapter' => 'PhutilOAuth1AuthAdapter',
+    'PhutilURIGoodie' => 'Phobject',
+    'PhutilURIGoodieTestCase' => 'PhabricatorTestCase',
     'PhutilWordPressAuthAdapter' => 'PhutilOAuthAuthAdapter',
     'PhutilXHPASTSyntaxHighlighter' => 'Phobject',
     'PhutilXHPASTSyntaxHighlighterFuture' => 'FutureProxy',
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
@@ -44,16 +44,15 @@
   protected function renderHyperlink($link, $name) {
     $engine = $this->getEngine();
 
-    $is_anchor = false;
-    if (strncmp($link, '/', 1) == 0) {
+    $uri = new PhutilURIGoodie($link);
+    $is_anchor = $uri->isAnchor();
+    if ($uri->isStartingWithSlash()) {
+      $here = $engine->getConfig('uri.here');
+      $link = $here.$link;
+    } else if ($is_anchor) {
       $base = phutil_string_cast($engine->getConfig('uri.base'));
       $base = rtrim($base, '/');
       $link = $base.$link;
-    } else if (strncmp($link, '#', 1) == 0) {
-      $here = $engine->getConfig('uri.here');
-      $link = $here.$link;
-
-      $is_anchor = true;
     }
 
     if ($engine->isTextMode()) {
@@ -76,7 +75,8 @@
       return $name;
     }
 
-    $same_window = $engine->getConfig('uri.same-window', false);
+    $is_internal = $uri->isInternal();
+    $same_window = $engine->getConfig('uri.same-window', $is_internal);
     if ($same_window) {
       $target = null;
     } else {
@@ -92,7 +92,7 @@
       'a',
       array(
         'href' => $link,
-        'class' => 'remarkup-link',
+        'class' => $this->getRemarkupLinkClass($is_internal),
         '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
@@ -116,7 +116,9 @@
 
     $engine = $this->getEngine();
 
-    $same_window = $engine->getConfig('uri.same-window', false);
+    $uri = new PhutilURIGoodie($link);
+    $is_internal = $uri->isInternal();
+    $same_window = $engine->getConfig('uri.same-window', $is_internal);
     if ($same_window) {
       $target = null;
     } else {
@@ -127,7 +129,7 @@
       'a',
       array(
         'href' => $link,
-        'class' => 'remarkup-link',
+        'class' => $this->getRemarkupLinkClass($is_internal),
         'target' => $target,
         'rel' => 'noreferrer',
       ),
diff --git a/src/infrastructure/markup/markuprule/PhutilRemarkupRule.php b/src/infrastructure/markup/markuprule/PhutilRemarkupRule.php
--- a/src/infrastructure/markup/markuprule/PhutilRemarkupRule.php
+++ b/src/infrastructure/markup/markuprule/PhutilRemarkupRule.php
@@ -106,4 +106,20 @@
     return (strpos($text, PhutilRemarkupBlockStorage::MAGIC_BYTE) === false);
   }
 
+  /**
+   * Get the CSS class="" attribute for a Remarkup link.
+   * It's just "remarkup-link" for all cases, plus the possibility for
+   * designers to style external links differently.
+   * @param  boolean $is_internal Whenever the link was internal or not.
+   * @return string
+   */
+  protected function getRemarkupLinkClass($is_internal) {
+    // Allow developers to style esternal links differently
+    $classes = array('remarkup-link');
+    if (!$is_internal) {
+      $classes[] = 'remarkup-link-ext';
+    }
+    return implode(' ', $classes);
+  }
+
 }
diff --git a/src/infrastructure/parser/PhutilURIGoodie.php b/src/infrastructure/parser/PhutilURIGoodie.php
new file mode 100644
--- /dev/null
+++ b/src/infrastructure/parser/PhutilURIGoodie.php
@@ -0,0 +1,138 @@
+<?php
+
+/**
+ * Wrapper for the PhutilURI class to be aware of the relative/absolute context
+ * and maybe other things in the future.
+ */
+final class PhutilURIGoodie extends Phobject {
+
+  /**
+   * Dependency injection with the URI object.
+   * @var PhutilURI
+   */
+  private $uriObj;
+
+  /**
+   * The original URI, untouched.
+   * @var string
+   */
+  private $uriOriginalStr;
+
+  /**
+   * Base URI of Phorge itself, as handy object.
+   * The base URI is supposed to never change across requests. So, static.
+   * Possible values:
+   *   - null:      Startup. Empty cache.
+   *   - PhutilURI: Cached value.
+   *   - false:     Cached no value.
+   * @var PhutilURI|null|false
+   */
+  private static $siteBaseUriObj;
+
+  /**
+   * Constructor by string.
+   * @param string $uri Partial or complete URI.
+   */
+  public function __construct($uri) {
+    $this->uriObj = new PhutilURI($uri);
+    $this->uriOriginalStr = $uri;
+  }
+
+  /**
+   * Get the original URI as string.
+   * @return string
+   */
+  public function getOriginalURI() {
+    return $this->uriOriginalStr;
+  }
+
+  /**
+   * Get the full URI as string.
+   * @return object
+   */
+  public function getURI() {
+    return $this->uriObj;
+  }
+
+  /**
+   * Check whenever an URI *is* just a simple HTML anchor.
+   * @param string $uri
+   * @return bool
+   */
+  public function isAnchor() {
+    return $this->isStartingWithChar('#');
+  }
+
+  /**
+   * Check whenever an URI starts with a slash (no protocol, etc.)
+   * @param string $uri
+   * @return bool
+   */
+  public function isStartingWithSlash() {
+    return $this->isStartingWithChar('/');
+  }
+
+  /**
+   * Check if this URI points to Phorge itself.
+   * This is a simple compromise between performance and a sane logic.
+   * At the moment we don't read the crystal ball, so these are
+   * considered different websites:
+   * - http://example.com (the input)
+   * - https://example.com (the base-uri)
+   * @param string $uri Example 'http://example.com/bar/'
+   * @return bool
+   */
+  public function isInternal() {
+
+    // If this URL has not a domain, indeed its the current domain.
+    // Probably this is a simple anchor, or just '/something/' etc.
+    $uri = $this->uriObj;
+    if ($uri->getProtocol() === '' && $uri->getDomain() === '') {
+      return true;
+    }
+
+    // Assume a safe default in case of missing base URI.
+    $base = self::baseURIObj();
+    if (!$base) {
+      return false;
+    }
+
+    return $uri->getDomain()   === $base->getDomain()
+        && $uri->getProtocol() === $base->getProtocol()
+        && $uri->getPort()     === $base->getPort();
+  }
+
+  /**
+   * Get a fresh URI object, cached.
+   * This is safe to be called a lot of times.
+   * @return PhutilURI|null
+   */
+  public static function baseURIObj() {
+    if (self::$siteBaseUriObj === null) {
+      self::$siteBaseUriObj = self::baseURIObjUncached();
+    }
+    return self::$siteBaseUriObj;
+  }
+
+  /**
+   * Get a fresh URI object (if any), uncached.
+   * @return PhutilURI|false
+   */
+  private static function baseURIObjUncached() {
+      $base = PhabricatorEnv::getEnvConfigIfExists('phabricator.base-uri');
+      if ($base) {
+        return new PhutilURI($base);
+      }
+      return false;
+  }
+
+  /**
+   * Check whenever the URI starts with the provided character.
+   * @param string $char String that MUST be of length = 1.
+   * @return boolean
+   */
+  private function isStartingWithChar($char) {
+    return strncmp($this->uriOriginalStr, $char, 1) === 0;
+  }
+
+}
diff --git a/src/infrastructure/parser/__tests__/PhutilURIGoodieTestCase.php b/src/infrastructure/parser/__tests__/PhutilURIGoodieTestCase.php
new file mode 100644
--- /dev/null
+++ b/src/infrastructure/parser/__tests__/PhutilURIGoodieTestCase.php
@@ -0,0 +1,34 @@
+<?php
+
+final class PhutilURIGoodieTestCase extends PhabricatorTestCase {
+
+  public function testURIInternal() {
+    $tests = array(
+      // test name, value, expected "is internal?"
+      array('internal anchor', '#asd', true),
+      array('internal relative dir', '/foo/', true),
+      array('internal relative dir also', 'foo/', true),
+      array('internal root dir', '/', true),
+      array('external uri', 'https://gnu.org/', false),
+    );
+
+    // If the base URI is set, also try against ourself.
+    $base_uri =
+      PhabricatorEnv::getEnvConfigIfExists('phabricator.base-uri');
+    if ($base_uri) {
+      // test name, value, expected "is internal?"
+      $tests[] = array('base uri', $base_uri, true);
+    }
+
+    foreach ($tests as $test) {
+      $test_name = $test[0];
+      $test_value = $test[1];
+      $test_expected = $test[2];
+
+      $tested_value = id(new PhutilURIGoodie($test_value))
+        ->isInternal();
+
+      $this->assertEqual($tested_value, $test_expected, $test_name);
+    }
+  }
+}