_wp_normalize_relative_css_links( string $css, string $stylesheet_url )

function _wp_normalize_relative_css_links( $css, $stylesheet_url ) {
	$has_src_results = preg_match_all( '#url\s*\(\s*[\'"]?\s*([^\'"\)]+)#', $css, $src_results );
	if ( $has_src_results ) {
		// Loop through the URLs to find relative ones.
		foreach ( $src_results[1] as $src_index => $src_result ) {
			// Skip if this is an absolute URL.
			if ( 0 === strpos( $src_result, 'http' ) || 0 === strpos( $src_result, '//' ) ) {
				continue;
			}

			// Skip if the URL is an HTML ID.
			if ( str_starts_with( $src_result, '#' ) ) {
				continue;
			}

			// Skip if the URL is a data URI.
			if ( str_starts_with( $src_result, 'data:' ) ) {
				continue;
			}

			// Build the absolute URL.
			$absolute_url = dirname( $stylesheet_url ) . '/' . $src_result;
			$absolute_url = str_replace( '/./', '/', $absolute_url );
			// Convert to URL related to the site root.
			$relative_url = wp_make_link_relative( $absolute_url );

			// Replace the URL in the CSS.
			$css = str_replace(
				$src_results[0][ $src_index ],
				str_replace( $src_result, $relative_url, $src_results[0][ $src_index ] ),
				$css
			);
		}
	}

	return $css;
}