Validation of this theme at http://d-theme.com/stark shows just one minor error where the XHTML does not match the schema for XHTML. There should not be any white space at the start or end of an attribute's value. This error is common to many themes and makes automated validation reject the theme.
<div id="navigation" class="menu withprimary withsecondary ">

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JohnAlbin’s picture

JohnAlbin
he/they/他
Primary language English
Location Taipei, Taiwan
CreditAttribution: JohnAlbin commented
Status: Active » Closed (won't fix)

Stark provides a window into Drupal 6’s default HTML. Start does not modify or provide any HTML of its own.

So the validation error you mention is due to Drupal core’s modules/system/page.tpl.php file.