Posting to a Drupal Form via HTML page
A form directly can not be posted to Drupal. But a general callback can receive post request data and can process it to store it somewhere.

For this, a hook_menu entry is required to register a callback with a newly created URL . This callback can access $_POST. 

On the other hand post request to Drupal can be invoked with form data either by CURL or jQuery.ajax()

Your custom module:

function hook_menu(){
  return array(
     'path/to/submit/post/values' =&gt; array(
        'title' =&gt; 'Catch Post Data',
        'access arguments' =&gt; array('access content'),
        'page callback' =&gt; 'catch_post_data',
        'type' =&gt; MENU_CALLBACK,
     ) 
  );
}

function catch_post_data(){
   $values = $_POST;
   // Get post values and store where you like.
   // Do not break the system. Do it carefully using proper entity functions.
   if($success)
       drupal_json(array('status' =&gt; 'SUCCESS'));
   else
       drupal_json(array('status' =&gt; 'FAILURE'));
}


Form can be posted via jQuery.

$.post('path/to/submit/post/values', $('#form').serialize(), function(response){
    response = $.parseJSON(response);
    if(response.status == 'SUCCESS')
        alert('Congrats!');
    else
        alert('Something went wrong');
});