Remove extra <br /> tag for XHTML strict validation in file question/preview.php.
Validator error shown:
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Line 130, Column 5: document type does not allow element "br" here; missing one of "p", "h1", "h2", "h3", "h4", "h5", "h6", "div", "pre", "address", "fieldset", "ins", "del" start-tag.
<br /><div class="controls"><input type="hidden" name="id" value="164" />
The mentioned element is not allowed to appear in the context in which you've placed it;
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patch file attached against current moodle 1.8