The HTML5 spec should specifically state that any field that validates input should allow the user to exit the field. At minimum, allow them to exit with the field being empty or retaining its initial value. Even though pattern validation is supposed to take place at submit time rather than during data entry or on navigation, the spec should be reviewed to see if the wording can be strengthened.

Use case: Svetlana is tabbing through the controls on a form and lands on a field that expects a telephone number, but when she tries to tab away the user agent puts up an error message saying that a valid telephone number is required. Even though she had no intention of completing the form, she is stuck until she makes up and enters a telephone number.

Use case: Etta brings up a web form showing her account information. As she tabs between the fields she lands on one containing her current password. Unfortunately, the security requirements for this web site have recently been increased and her password is no longer considered secure enough, so even though she's only tabbing through the fields, the current value fails to validate, and she is prevented from tabbing on with its current value or by clearing the field.