As you know, the osTube is using XHTML 1.0 Strict
but I have found a couple files which break those standards.
These have been found is version 2.3
(For version 2.2, see bottom of post)
Also, with each template change, arises different errors.
I just went through version 2.3 and performed the following to meet standards.

1.) includes/boxes/search.php
At or around Line 28
onClick='this.focus();this.select();
must be
onclick='this.focus();this.select();
since XHTML 1.0 Strict requires attributes to be lower case.

2.) templates/default/index.tpl
Line 88
<a href="http://www.ostube.de" target="_blank" id="ostubelink" style="width:4em;" id="ostubelink">

a.) The target attribute is depreciated in XHTML 1.0 Strict
b.) There are duplicate ID attributes

Solution: Remove target="_blank" id="ostubelink"
leaving the end id="ostubelink" intact so that your line reads:
<a href="http://www.ostube.de" style="width:4em;" id="ostubelink">

Line 89
<script language="JavaScript" type="text/javascript">
The language attribute is depreciated in favor of type.
Solution: Remove language="JavaScript"
so that your line reads:
<script type="text/javascript">

Version 2.2
If you wish to get your osTube 2.2 default or hightek back into compliance, see below:

1.)includes/boxes/search.php
At or around Line 30
onClick='this.focus();this.select();
must be
onclick='this.focus();this.select();
since XHTML 1.0 Strict requires attributes to be lower case.
(Applies also to latest version 2.3)

2.)templates/default/home_media.tpl
At or around Line 35
so.getVariablePairs().join("&");
must be
so.getVariablePairs().join("&");
since you are trying to parse an unescaped ampersand.

If you are interested in validating your own site,
try visiting http://validator.w3.org/