Some websites use legal-like jargon to cause users to not opt-out (i.e opt-in). They use the word not in their prompts. As a programmer you always pay close attention to when the not operator is applied to an expression.

Yes, I do not want to not opt-in.

Note: JavaScript is used to cause the checkbox input element to be automatically checked and unchecked -- i.e. it blinks. [ view source]

Yes, I do want to opt-in.

The default is opt-out. The checkbox input element is never automatically checked. If the user wants to opt-in, then they check the box. The prompt does not contain the word not.