In admin\includes\languages\english\admin_activity.php
regarding the last define

1)
PHP Code:
define('TEXT_INTERPRETING_LOG_DATA', '<p><strong>Interpretation of the log data</strong><ul>... 
has a list inside a p tag, which is invalid html.

Suggest closing the p or making it a h2 or h3 heading
PHP Code:
define('TEXT_INTERPRETING_LOG_DATA', '<p><strong>Interpretation of the log data</strong></p><ul>... 
The final/orphan </p> of the define should consequently be removed.

2) the nested list inside the containing li does nto have a closing li:
PHP Code:
...<li><strong>WARNING</strong> is assigned to CRITICAL things such as removal of payment modules or deletion of admin users. These are activities which might suggest pending trouble if not caught quickly. These should be reviewed very frequently; recommended daily.</li></ul><li> 
suggest
PHP Code:
...<li><strong>WARNING</strong> is assigned to CRITICAL things such as removal of payment modules or deletion of admin users. These are activities which might suggest pending trouble if not caught quickly. These should be reviewed very frequently; recommended daily.</li></ul></li><li> 
----------------
In admin\admin_activity.php

1) The doctype puts all browsers in quirks mode. Not necessarily an error, but not a good thing?

It uses this
PHP Code:
<?php //echo '<!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN">'; ?>
suggest this
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
2) The form declaration is outside the <td>tags: invalid.

From
<?php if ($action == '') { ?>
<tr><?php echo zen_draw_form('export', FILENAME_ADMIN_ACTIVITY, 'action=save', 'post'); //, 'onsubmit="return check_form(export);"'); ?>
<td align="center">
<tableThe border="0" cellspacing="0" cellpadding="2">
to
<?php if ($action == '') { ?>
<tr>
<td align="center">
<?php echo zen_draw_form('export', FILENAME_ADMIN_ACTIVITY, 'action=save', 'post'); //, 'onsubmit="return check_form(export);"'); ?>
<table border="0" cellspacing="0" cellpadding="2">
From
</table>
</td>
</form>
</tr>
to
</table>
</form>
</td>
</tr>

3) the body table tags are unbalanced.

From
<?php } ?>
<!-- body_text_eof //-->
</table>
<!-- body_eof //--> <!-- footer //-->
to
<?php } ?>
</table>
</td>
<!-- body_text_eof //-->
</tr>
</table>
<!-- body_eof //--> <!-- footer //-->