if ( empty( $tagnames ) ) {
    return $content;
}

// line 196 --v
$content = do_shortcodes_in_html_tags( $content, $ignore_html, $tagnames );

function first_shortcode() {
    $content = '<h1>This is my first shortcode</h1>';
    return $content;
}
add_shortcode('my_shortcode', 'first_shortcode');