Hi,
First I want to clarify that I've never claimed mojoPortal validates with Xhtml Strict doctypes, and this border on the Subkismet captcha image is completely valid in Xhtml 1.0 Transitional and I get a green success with no warnings from W3c using Xhtml 1.0 Transitional.
Nevertheless, I have modified the source code of Subkismet to remove the border, so this border will not be there in the next release of mojoPortal.
Best,
Joe