Pass variables from html preprocess function to page preprocess function?
You're going to need either a global or static variable to accomplish this...personally I'd recommend going with static and implementing a generic helper function using Drupal's static pattern like so:

function _myTheme_var($var_name, $new_val = NULL) {
  $vars = &amp;drupal_static(__FUNCTION__, array());

  // If a new value has been passed
  if ($new_val) {
    $vars[$var_name] = $new_val;
  }

  return isset($vars[$var_name]) ? $vars[$var_name] : NULL;
}

function myTheme_preprocess_page(&amp;$vars){
  $vars['some_var'] = _myTheme_var('my_var', 7); // Set the value and retrieve it
}

function myTheme_preprocess_html(&amp;$vars){
  $vars['some_var'] = _myTheme_var('my_var'); // Get the value
}


The $vars arrays for each preprocess function are completely different objects in memory so you won't be able to use that array to pass things around. I've seen this sort of pattern used a lot in Drupal core so I think it would be the best way to go.

EDIT

Oh and yes, hook_preprocess_page() is called before hook_preprocess_html()...as far as I'm aware Drupal works in a sort of "inside-out" manner so hook_preprocess_html() would probably be the last one that's run in a page build.