diff --git a/includes/parser/Sanitizer.php b/includes/parser/Sanitizer.php
index fbcd316c1ba..a54bf5959b3 100644
--- a/includes/parser/Sanitizer.php
+++ b/includes/parser/Sanitizer.php
@@ -1291,7 +1291,7 @@ class Sanitizer {
 
 		$mode = $wgFragmentMode[self::ID_PRIMARY];
 
-		$id = self::escapeIdInternal( $id, $mode );
+		$id = self::escapeIdInternalUrl( $id, $mode );
 
 		return $id;
 	}
@@ -1308,11 +1308,28 @@ class Sanitizer {
 	public static function escapeIdForExternalInterwiki( $id ) {
 		global $wgExternalInterwikiFragmentMode;
 
-		$id = self::escapeIdInternal( $id, $wgExternalInterwikiFragmentMode );
+		$id = self::escapeIdInternalUrl( $id, $wgExternalInterwikiFragmentMode );
 
 		return $id;
 	}
 
+	/**
+	 * Do percent encoding of percent signs for href (but not id) attributes
+	 *
+	 * @since 1.35
+	 * @see https://phabricator.wikimedia.org/T238385
+	 * @param string $id String to escape
+	 * @param string $mode One of modes from $wgFragmentMode
+	 * @return string
+	 */
+	private static function escapeIdInternalUrl( $id, $mode ) {
+		$id = self::escapeIdInternal( $id, $mode );
+		if ( $mode === 'html5' ) {
+			$id = preg_replace( '/%([a-fA-F0-9]{2})/', '%25$1', $id );
+		}
+		return $id;
+	}
+
 	/**
 	 * Helper for escapeIdFor*() functions. Performs most of the actual escaping.
 	 *
diff --git a/tests/parser/parserTests.txt b/tests/parser/parserTests.txt
index 092cd3a75cb..5fb666f0ee1 100644
--- a/tests/parser/parserTests.txt
+++ b/tests/parser/parserTests.txt
@@ -22858,7 +22858,7 @@ wgFragmentMode=[ 'html5', 'legacy' ]
 __NOEDITSECTION__
 !! html/php
 <h3><span id=".2B:.3A.253A_.26.26.5D.5D_x"></span><span class="mw-headline" id="+:.3A%3A_&amp;&amp;]]_x">_ +:.3A%3A _ &amp;&amp;]] x</span></h3>
-<p>+:.3A%3A_&amp;&amp;&#93;&#93;_x
+<p>+:.3A%253A_&amp;&amp;&#93;&#93;_x
 </p>
 !! html/parsoid
 <h3 id="+:.3A%3A_&amp;&amp;]]_x"><span id=".2B:.3A.253A_.26.26.5D.5D_x" typeof="mw:FallbackId"></span>_ +:.3A%3A _ &amp;<span typeof="mw:Entity" data-parsoid='{"src":"&amp;amp;","srcContent":"&amp;","dsr":[18,23,null,null]}'>&amp;</span>]] x</h3>
diff --git a/tests/phpunit/includes/parser/SanitizerTest.php b/tests/phpunit/includes/parser/SanitizerTest.php
index cff3d6d4347..176d02d7498 100644
--- a/tests/phpunit/includes/parser/SanitizerTest.php
+++ b/tests/phpunit/includes/parser/SanitizerTest.php
@@ -173,6 +173,7 @@ class SanitizerTest extends MediaWikiTestCase {
 	 * @covers Sanitizer::escapeIdForLink()
 	 * @covers Sanitizer::escapeIdForExternalInterwiki()
 	 * @covers Sanitizer::escapeIdInternal()
+	 * @covers Sanitizer::escapeIdInternalUrl()
 	 *
 	 * @param string $stuff
 	 * @param string[] $config
@@ -193,10 +194,11 @@ class SanitizerTest extends MediaWikiTestCase {
 
 	public function provideEscapeIdForStuff() {
 		// Test inputs and outputs
-		$text = 'foo тест_#%!\'()[]:<>&&amp;&amp;amp;';
+		$text = 'foo тест_#%!\'()[]:<>&&amp;&amp;amp;%F0';
 		$legacyEncoded = 'foo_.D1.82.D0.B5.D1.81.D1.82_.23.25.21.27.28.29.5B.5D:.3C.3E' .
-			'.26.26amp.3B.26amp.3Bamp.3B';
-		$html5Encoded = 'foo_тест_#%!\'()[]:<>&&amp;&amp;amp;';
+			'.26.26amp.3B.26amp.3Bamp.3B.25F0';
+		$html5EncodedId = 'foo_тест_#%!\'()[]:<>&&amp;&amp;amp;%F0';
+		$html5EncodedHref = 'foo_тест_#%!\'()[]:<>&&amp;&amp;amp;%25F0';
 
 		// Settings: last element is $wgExternalInterwikiFragmentMode, the rest is $wgFragmentMode
 		$legacy = [ 'legacy', 'legacy' ];
@@ -214,27 +216,27 @@ class SanitizerTest extends MediaWikiTestCase {
 
 			// Transition to a new world: legacy links with HTML5 fallback
 			[ 'Attribute', $legacyNew, $text, $legacyEncoded, Sanitizer::ID_PRIMARY ],
-			[ 'Attribute', $legacyNew, $text, $html5Encoded, Sanitizer::ID_FALLBACK ],
+			[ 'Attribute', $legacyNew, $text, $html5EncodedId, Sanitizer::ID_FALLBACK ],
 			[ 'Link', $legacyNew, $text, $legacyEncoded ],
 			[ 'ExternalInterwiki', $legacyNew, $text, $legacyEncoded ],
 
 			// New world: HTML5 links, legacy fallbacks
-			[ 'Attribute', $newLegacy, $text, $html5Encoded, Sanitizer::ID_PRIMARY ],
+			[ 'Attribute', $newLegacy, $text, $html5EncodedId, Sanitizer::ID_PRIMARY ],
 			[ 'Attribute', $newLegacy, $text, $legacyEncoded, Sanitizer::ID_FALLBACK ],
-			[ 'Link', $newLegacy, $text, $html5Encoded ],
+			[ 'Link', $newLegacy, $text, $html5EncodedHref ],
 			[ 'ExternalInterwiki', $newLegacy, $text, $legacyEncoded ],
 
 			// Distant future: no legacy fallbacks, but still linking to leagacy wikis
-			[ 'Attribute', $new, $text, $html5Encoded, Sanitizer::ID_PRIMARY ],
+			[ 'Attribute', $new, $text, $html5EncodedId, Sanitizer::ID_PRIMARY ],
 			[ 'Attribute', $new, $text, false, Sanitizer::ID_FALLBACK ],
-			[ 'Link', $new, $text, $html5Encoded ],
+			[ 'Link', $new, $text, $html5EncodedHref ],
 			[ 'ExternalInterwiki', $new, $text, $legacyEncoded ],
 
 			// Just before the heat death of universe: external interwikis are also HTML5 \m/
-			[ 'Attribute', $allNew, $text, $html5Encoded, Sanitizer::ID_PRIMARY ],
+			[ 'Attribute', $allNew, $text, $html5EncodedId, Sanitizer::ID_PRIMARY ],
 			[ 'Attribute', $allNew, $text, false, Sanitizer::ID_FALLBACK ],
-			[ 'Link', $allNew, $text, $html5Encoded ],
-			[ 'ExternalInterwiki', $allNew, $text, $html5Encoded ],
+			[ 'Link', $allNew, $text, $html5EncodedHref ],
+			[ 'ExternalInterwiki', $allNew, $text, $html5EncodedHref ],
 		];
 	}