inc/parsers/tidy.php
This parser removes HTML tags to prevent malicious actions. A small number of tags are preserved: b i u s pre sup sub tt code span br hr small big div
Also, the < > <> are preserved if they are not context of a tag. (They are used as operators for the query function.)
It replaces also & by its tag but preserves numbered entities.
Note that if you need HTML tags, you keep them using the nowiki tag. Text on a line inside a nowiki tag is not parsed at all. If you have multiple lines, you need to add a nowiki tag for each line.