Also, validation errors is how you get people to ignore XHTML. Strict validation is tricky. With HTTPS, validation was initially very lax, and AFAIK, most SMT

Also, validation errors is how you get people to ignore XHTML. Strict validation is tricky. With HTTPS, validation was initially very lax, and AFAIK, most SMTP servers still do not validate the certificates of the destination SMTP servers. It is hard to get people to move to stricter validation once habits and working systems have been established which depend on lax – or no – validation. Some try to avoid this by specifying strict validation in advance, like XHTML and DNSSEC, which obviously has its own drawbacks.