They say they follow the W3C standards and they do.
The page: http://www.opera.com/
This document was successfully checked as XHTML 1.0 Strict!Result: Passed
The uploaded document "default.htm" was successfully checked as XHTML 1.0 Strict. This means that the resource in question identified itself as "XHTML 1.0 Strict" and that we successfully performed a formal validation using an SGML or XML Parser (depending on the markup language used).
What about Microsoft?
Errors found while checking this document as XHTML 1.0 Transitional!Result: 176 Errors, 36 warning(s)
The site used for the checking: http://validator.w3.org/