Problem description:

Both student submission and teacher response are saved into the database correctly. So the HTML typed inside the html editor is stored straight into the database.
On the other hand, when this submission is shown it is printed by the text_format php function. The problem is that the HTML_FORMAT filter cleans the styles saved in the database(clean_text php function)

Possible Solution:

/*

• Teacher´s feedback
  *
• file=/lib/gradelib.php:450-452
  */

450 $options = new stdClass;
451 $options->noclean = true;
452 $grade->str_feedback = format_text($grade->feedback, $grade->feedbackformat,$options);

/*

• Student´s submission
  *
• file=/mod/assignment/type/online/assignment.class.php:104-106
  */

104 $options = new stdClass;
105 $options->noclean = true;
106 $grade->str_feedback = format_text($grade->feedback, $grade->feedbackformat,$options);

I´m not that sure how these modifications could affect to the moodle style page and if there is a posibility of malicious code injection through showing RAW html.