Strip <link rel="alternate"> from global header REST output

mu-plugins/blocks/global-header-footer/blocks.php

@@ -405,6 +405,12 @@ function render_global_header( $attributes = array() ) {
 		$head_markup = ob_get_clean();
 	}
 
+	// Strip <link rel="alternate"> tags from the head, as they're not relevant for the global header.
+	// This catches tags from core (feed links, oEmbed) and any added by plugins.
+	if ( $is_rest_request ) {
+		$head_markup = remove_head_alternate_links( $head_markup );
+	}
+
 	$wrapper_attributes = get_block_wrapper_attributes(
 		array( 'class' => 'global-header wp-block-group' )
 	);
@@ -417,6 +423,21 @@ function render_global_header( $attributes = array() ) {
 	);
 }
 
+/**
+ * Remove `<link rel="alternate" ...>` tags from HTML markup.
+ *
+ * Uses a regex to match and remove `<link>` tags whose `rel` attribute
+ * contains "alternate", regardless of which plugin or core function
+ * added them.
+ *
+ * @param string $markup The HTML markup to filter.
+ *
+ * @return string The markup with alternate link tags removed.
+ */
+function remove_head_alternate_links( $markup ) {
+	return preg_replace( '/<link\b[^>]*\brel=["\'][^"\']*\balternate\b[^"\']*["\'][^>]*\/?>/i', '', $markup );
+}
+
 /**
  * Determine if the current site is a Rosetta site (e.g., `es-mx.wordpress.org`).
  *

phpunit/test-global-header-footer.php

@@ -0,0 +1,127 @@
+<?php
+
+use function WordPressdotorg\MU_Plugins\Global_Header_Footer\remove_head_alternate_links;
+
+class Test_Global_Header_Footer extends WP_UnitTestCase {
+
+	/**
+	 * Test that RSS feed link tags are removed.
+	 */
+	public function test_removes_rss_feed_link() {
+		$input = '<link rel="alternate" type="application/rss+xml" title="Feed" href="https://example.com/feed/" />';
+		$this->assertEmpty( trim( remove_head_alternate_links( $input ) ) );
+	}
+
+	/**
+	 * Test that Atom feed link tags are removed.
+	 */
+	public function test_removes_atom_feed_link() {
+		$input = '<link rel="alternate" type="application/atom+xml" title="Atom" href="https://example.com/feed/atom/" />';
+		$this->assertEmpty( trim( remove_head_alternate_links( $input ) ) );
+	}
+
+	/**
+	 * Test that oEmbed discovery link tags are removed.
+	 */
+	public function test_removes_oembed_link() {
+		$input = '<link rel="alternate" type="application/json+oembed" href="https://example.com/wp-json/oembed/1.0/embed?url=test" />';
+		$this->assertEmpty( trim( remove_head_alternate_links( $input ) ) );
+	}
+
+	/**
+	 * Test that hreflang alternate link tags are removed.
+	 */
+	public function test_removes_hreflang_link() {
+		$input = '<link rel="alternate" hreflang="es" href="https://es.wordpress.org/" />';
+		$this->assertEmpty( trim( remove_head_alternate_links( $input ) ) );
+	}
+
+	/**
+	 * Test that alternate links with single-quoted attributes are removed.
+	 */
+	public function test_removes_single_quoted_alternate() {
+		$input = "<link rel='alternate' type='application/rss+xml' href='https://example.com/feed/' />";
+		$this->assertEmpty( trim( remove_head_alternate_links( $input ) ) );
+	}
+
+	/**
+	 * Test that alternate links without self-closing slash are removed.
+	 */
+	public function test_removes_non_self_closing() {
+		$input = '<link rel="alternate" type="application/rss+xml" href="https://example.com/feed/">';
+		$this->assertEmpty( trim( remove_head_alternate_links( $input ) ) );
+	}
+
+	/**
+	 * Test that compound rel values containing "alternate" are removed.
+	 */
+	public function test_removes_compound_rel_with_alternate() {
+		$input = '<link rel="alternate stylesheet" title="Dark" href="dark.css" />';
+		$this->assertEmpty( trim( remove_head_alternate_links( $input ) ) );
+	}
+
+	/**
+	 * Test that stylesheet links are preserved.
+	 */
+	public function test_preserves_stylesheet_link() {
+		$input = '<link rel="stylesheet" href="style.css">';
+		$this->assertSame( $input, remove_head_alternate_links( $input ) );
+	}
+
+	/**
+	 * Test that canonical links are preserved.
+	 */
+	public function test_preserves_canonical_link() {
+		$input = '<link rel="canonical" href="https://example.com/page">';
+		$this->assertSame( $input, remove_head_alternate_links( $input ) );
+	}
+
+	/**
+	 * Test that preconnect links are preserved.
+	 */
+	public function test_preserves_preconnect_link() {
+		$input = '<link rel="preconnect" href="https://fonts.googleapis.com">';
+		$this->assertSame( $input, remove_head_alternate_links( $input ) );
+	}
+
+	/**
+	 * Test that icon links are preserved.
+	 */
+	public function test_preserves_icon_link() {
+		$input = '<link rel="icon" href="/favicon.ico">';
+		$this->assertSame( $input, remove_head_alternate_links( $input ) );
+	}
+
+	/**
+	 * Test that mixed markup keeps non-alternate links and removes alternate ones.
+	 */
+	public function test_mixed_markup() {
+		$input = '<head>
+<link rel="alternate" type="application/rss+xml" title="Feed" href="/feed/" />
+<link rel="stylesheet" href="style.css">
+<link rel="alternate" type="application/json+oembed" href="/oembed" />
+<link rel="preconnect" href="https://fonts.googleapis.com">
+</head>';
+
+		$result = remove_head_alternate_links( $input );
+
+		$this->assertStringContainsString( 'stylesheet', $result );
+		$this->assertStringContainsString( 'preconnect', $result );
+		$this->assertStringNotContainsString( 'alternate', $result );
+	}
+
+	/**
+	 * Test that empty string input returns empty string.
+	 */
+	public function test_empty_input() {
+		$this->assertSame( '', remove_head_alternate_links( '' ) );
+	}
+
+	/**
+	 * Test that markup with no link tags is returned unchanged.
+	 */
+	public function test_no_link_tags() {
+		$input = '<div class="header">Hello</div>';
+		$this->assertSame( $input, remove_head_alternate_links( $input ) );
+	}
+}