Running v1.5.1. For whatever reason, I chose to view the HTML source after a DTK search and found that the search results are created before the <!doctype> tag:
Code:
<table border="0" width="100%" cellspacing="2" cellpadding="1" align="center">
<tr class="infoBoxContent"><td class="dataTableHeadingContent">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Searching 1474 files ... for: slkdfj</td></tr></table>

<tr><td>&nbsp;</td></tr><table border="0" width="100%" cellspacing="2" cellpadding="1" align="center"><tr class="infoBoxContent"><td class="dataTableHeadingContent">Match Lines found: 0</td></tr></table><!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN">
<html dir="ltr" lang="en">
<head>
While it's not (to my knowledge) causing any problems today, it should probably be corrected to product valid HTML in the future.