diff --git a/src/wp-includes/customize/class-wp-customize-custom-css-setting.php b/src/wp-includes/customize/class-wp-customize-custom-css-setting.php
index aab0e475304ea..0d8324e367a3a 100644
--- a/src/wp-includes/customize/class-wp-customize-custom-css-setting.php
+++ b/src/wp-includes/customize/class-wp-customize-custom-css-setting.php
@@ -144,35 +144,6 @@ public function value() {
 		return $value;
 	}
 
-	/**
-	 * Validate a received value for being valid CSS.
-	 *
-	 * Checks for imbalanced braces, brackets, and comments.
-	 * Notifications are rendered when the customizer state is saved.
-	 *
-	 * @since 4.7.0
-	 * @since 4.9.0 Checking for balanced characters has been moved client-side via linting in code editor.
-	 * @since 5.9.0 Renamed `$css` to `$value` for PHP 8 named parameter support.
-	 *
-	 * @param string $value CSS to validate.
-	 * @return true|WP_Error True if the input was validated, otherwise WP_Error.
-	 */
-	public function validate( $value ) {
-		// Restores the more descriptive, specific name for use within this method.
-		$css = $value;
-
-		$validity = new WP_Error();
-
-		if ( preg_match( '#</?\w+#', $css ) ) {
-			$validity->add( 'illegal_markup', __( 'Markup is not allowed in CSS.' ) );
-		}
-
-		if ( ! $validity->has_errors() ) {
-			$validity = parent::validate( $css );
-		}
-		return $validity;
-	}
-
 	/**
 	 * Store the CSS setting value in the custom_css custom post type for the stylesheet.
 	 *
diff --git a/src/wp-includes/theme.php b/src/wp-includes/theme.php
index 0ff915cbe4263..bb6498f23dbae 100644
--- a/src/wp-includes/theme.php
+++ b/src/wp-includes/theme.php
@@ -2144,6 +2144,13 @@ function wp_update_custom_css_post( $css, $args = array() ) {
 
 	// Update post if it already exists, otherwise create a new one.
 	$post = wp_get_custom_css_post( $args['stylesheet'] );
+
+	// Remove KSES HTML filters to prevent CSS mangling.
+	$priority = has_filter( 'content_save_pre', 'wp_filter_post_kses' );
+	if ( false !== $priority ) {
+		remove_filter( 'content_save_pre', 'wp_filter_post_kses', $priority );
+	}
+
 	if ( $post ) {
 		$post_data['ID'] = $post->ID;
 		$r               = wp_update_post( wp_slash( $post_data ), true );
@@ -2163,6 +2170,10 @@ function wp_update_custom_css_post( $css, $args = array() ) {
 		}
 	}
 
+	if ( false !== $priority ) {
+		add_filter( 'content_save_pre', 'wp_filter_post_kses', $priority );
+	}
+
 	if ( is_wp_error( $r ) ) {
 		return $r;
 	}
diff --git a/tests/phpunit/tests/customize/custom-css-setting.php b/tests/phpunit/tests/customize/custom-css-setting.php
index 65cc3f717fe59..e478f73a41c3f 100644
--- a/tests/phpunit/tests/customize/custom-css-setting.php
+++ b/tests/phpunit/tests/customize/custom-css-setting.php
@@ -268,6 +268,27 @@ public function test_get_custom_css_post_queries_after_failed_lookup() {
 		$this->assertSame( get_num_queries(), $queries_before );
 	}
 
+	/**
+	 * Ensure that dangerous STYLE tag contents do not break HTML output.
+	 *
+	 * @ticket 64418
+	 */
+	public function test_wp_custom_css_cb_escapes_dangerous_html() {
+		wp_update_custom_css_post(
+			'*::before { content: "</style><script>alert(1)</script>"; }',
+			array(
+				'stylesheet' => $this->setting->stylesheet,
+			)
+		);
+		$output   = get_echo( 'wp_custom_css_cb' );
+		$expected = <<<'HTML'
+<style id="wp-custom-css" type="text/css">
+*::before { content: "\3c\2fstyle><script>alert(1)</script>"; }
+</style>
+HTML;
+		$this->assertEqualHTML( $expected, $output );
+	}
+
 	/**
 	 * Test that wp_update_custom_css_post() updates the 'custom_css_post_id' theme mod.
 	 *
@@ -373,29 +394,4 @@ public function filter_update_custom_css_data( $data, $args ) {
 		$data['post_title']   = 'Ignored';
 		return $data;
 	}
-
-	/**
-	 * Tests that validation errors are caught appropriately.
-	 *
-	 * Note that the $validity \WP_Error object must be reset each time
-	 * as it picks up the Errors and passes them to the next assertion.
-	 *
-	 * @covers WP_Customize_Custom_CSS_Setting::validate
-	 */
-	public function test_validate() {
-
-		// Empty CSS throws no errors.
-		$result = $this->setting->validate( '' );
-		$this->assertTrue( $result );
-
-		// Basic, valid CSS throws no errors.
-		$basic_css = 'body { background: #f00; } h1.site-title { font-size: 36px; } a:hover { text-decoration: none; } input[type="text"] { padding: 1em; }';
-		$result    = $this->setting->validate( $basic_css );
-		$this->assertTrue( $result );
-
-		// Check for markup.
-		$unclosed_comment = $basic_css . '</style>';
-		$result           = $this->setting->validate( $unclosed_comment );
-		$this->assertArrayHasKey( 'illegal_markup', $result->errors );
-	}
 }