Do not let a form field receiving focus or being filled in trigger a major event.

Do not allow a major event to occur when a visitor fills in an input field, clicks a check box or radio button, or chooses a value from a drop-down list, unless the visitor has been informed beforehand. For many visitors, an unexpected event can be confusing. Visually impaired visitors may not be able to see the change. As a result, visitors who only navigate with the keyboard may have difficulty operating the website. Therefore, let the visitor click on a button to trigger the event.

Major events are called context changes. The context changes are for example:

There should also be no change of context by changing the focus.