Re: XHTML validator fail (just 1 error)
Markup validation or malformed markup does not really correlate to lack of security.
Whereas WP strove to produce valid markup that would pass validation against DTD schemas there was always a fundamental flaw to maintaining that valid code which is third party coding i.e plugins, sadly when you allow third parties to introduce code/markup to the application you lose control over this aspect as plugin authors will have varying degrees of proficiency where frontend coding is concerned.
BP by and large – like WP – produces valid acceptable markup (we’ll overlook empty clearing divs ) however as one starts to add the vital and necessary plugins into the mix this valid code very rapidly falls apart. You can spend a lot of time if you so choose correcting these plugins, chasing down a heap of markup errors, but it will be a somewhat never ending task and not really that productive a use of someones time. Currently looking at our index page shows me 33 errors mainly produced by a video sidebar widget, most of these errors could be corrected, doing so in this instance largely a waste of time.
Interestingly /activity/ directory produces a fully compliant page, what page are you seeing your error on?