This form allows you to check a web page against HTML Tidy. By entering a URL, you will get the page directly from the web server, and any server-side processing (chtml, PHP, etc.) will be done before Tidy is used.
Since the source passed to Tidy might be different than the source on the web server (for example you may use chtml or PHP, or be generating output from a CGI), the line numbers could differ from your source. Also beware that errors may actually be in included files or generated HTML.