We are passionate about producing websites that comply with the W3C Standards and are therefore error free. In other words, each page is tested and validated on the
W3C Validator and has to achieve a 100% pass.
The advantages of this is that the website will be more future-proof. At the moment desktop web browsers are very tolerant of errors, but this will change. Mobile phone browsers tend to be very intolerant and therefore a page that is not validated is unlikely to display at all.
Also many accessibility aids require validated code to operate.
As a programmer, if this bothered me enough I’d contribute time to fixing it, but frankly I don’t care.