return page of mostly static html

A module I'm developing needs to generate a report based on information submitted in a user form. While the report does have some dynamic elements, the bulk of the page is static. At the moment, I'm writing in the function itself, but that's really ugly and not exactly designer friendly. I'd like to move it out into it's own file. What's the recommended practice for getting that into the a format returnable by the page function?

/**
*  Displays the score report.
*
*  @return
*    HTML to be displayed
*/
function who5_score_report() {
  //get data from cookie
  $score_array = unserialize($_COOKIE['WHO5']);

  //get raw score
  $total_score = array_sum($score_array['scores']);
  $pct_score = $total_score*4 . "%";

  //set score based display parameters
  //TODO: make color configurable
  if ($total_score > 18) {
    $score_color = "green";
  }
  else if ($total_score <= 13) {
    $score_color = "red";
  }
  else {
    $score_color = "yellow";
  }

  //check individual scores for problems (i.e., any score of 0 or 1)
  $failed_question = array();
  foreach ($score_array['scores'] as $i => $score) {
    if ($score <= 1) //any individual score of 1 or 0 represents a warning sign
      $failed_questions[] = $i;
  }

  //create html to ouput
  $output = "<style type=\"text/css\">\n#who5header{text-align:center;margin-bottom:20px}\n.who5score{color:$score_color}\n</style>\n";
  $output .= "<div id=\"who5header\"><h1>" . t('WHO-5 Score Report') . "</h1></div>\n";
  $output .= "<div id=\"who5scoreSummary\"><h2>Summary</h2>\n";
  $output .= t('Assessment completed: ' . date('m-d-Y h:i:s', $score_array['completion_time'])) . "<br />";
  $output .= t('Raw Score: <span class="who5score">%score</span> (<span class="who5score">%pct_score</span>)', array('%score' => $total_score, '%pct_score' => $pct_score));
  $output .= "</div><!--closing who5scoreSummary-->\n";
  $output .= "<div>" . print_r($failed_questions, TRUE) . "</div>";
  $output .= "<div>" . print_r($score_array, TRUE) . "</div>";
  return $output;
}