This can also be seen in the demo page: http://demo.moodle.org/mod/scorm/player.php?a=4&scoid=25

The main problems are:

1) The form used here is
<form name="tochead" method="post" action="player.php?id=237" target="_top">
<input name="prev" type="button" value="Previous" onClick="document.location.href=' http://demo.moodle.org/mod/scorm/player.php?id=237&amp;currentorg=&amp;mode=normal&amp;scoid=24 '"/>
</form>
The form tag does not allow the parameters "name" and "target" in XHTML 1.0 Strict. Instead we should use "id" and no targe attribute.
The input tag does not allow "onClick". The task of changing to url should be done in the "action" attribute of the form.

2) All spacers image require an "alt" attribute.
<img src="http://demo.moodle.org/mod/scorm/pix/spacer.gif" />
should be
<img src="http://demo.moodle.org/mod/scorm/pix/spacer.gif" alt="" />
For more info why the alt attribute should be empty, visit: http://diveintoaccessibility.org/day_21_ignoring_spacer_images.html
Moreover, this kind of web design should be avoided and use CSS instead.

3) The SCORM player uses iframes which are not supported by XHTML 1.0 Strict. We should use the object tag instead as discussed in: MDL-17702