Описание тега checked
The checked attribute of the HTML input tag used to set the checked state of the control to “on” by default. Used to set the state of the input controls with the type attribute "checkbox" or "radio".
Used to present one or more checkboxes in such a way that the state of the control at the point of page load is checked, and the user has to opt out rather than opting in, or perhaps presenting the user with a form — including a series of checkboxes — that he or she has previously filled in, and preserve the state of the selections that were made. The checked attribute allows you to set the checked state to “on” by default.
The checked attribute is also used for radio input controls. Unlike the checkbox, only one in a range of related radio inputs can be selected, and the checked attribute is used to identify which one is selected. If you accidentally mark as "checked" a number of radio inputs that share a given name attribute, the last one that’s marked as such will be selected.