You can cite the W3C as your source, here and here.
Essentially a radio button is a group that functions as a single element since it retains only a single value. Tabbing to a radio group will bring you to the first item and then using the arrow keys you navigate within the group.
- Focus can move to a radio group via:
- The Tab key
- An Access Key or mnemonic targeting the label
- Activation of the label (through Assistive Technology mechanism)
- The Tab key moves focus between radio button
groups and other widgets.- When focus is on the group and
when no radio button is selected:
- Tab key press moves focus
to the first radio button in the group, but does not select the radio
button.- Shift+Tab key press moves focus to the last radio
button in the group, but does not select the radio button.- When focus moves to the group in which a radio button is
selected, pressing Tab and Shift+Tab keys move focus to the radio
button that is checked.- Up Arrow and Down Arrow keys move
focus and selection.
- Up Arrow key press moves focus and
selection forward through the radio buttons in the group. If the first
radio button has focus, then focus and selection move to the last
radio button in the group.- Down Arrow key press moves focus
and selection backward through the radio buttons in the group. If the
last radio button has focus, then focus and selection move to the
first radio button in the group. * Space Bar key press checks the
radio button that currently has focus.- When
re-entering the group, focus returns to the point of previous focus
(the item with selection set).- Pressing the Tab key exits
the group and moves focus to the next form control.- Pressing
the Shift+Tab keys exits the group and moves focus to the previous
form control.