Emulating tabbing to radio buttons should focus the selected radio button, if any

[joelpurra]

Spotted in the PlusAsTab enter-as-tab demo using Chrome 18.

Expected behavior

1. Click "Radio 2" to select radio button number two.
2. Focus the number input field ("999").
3. Press tab once. Notice that radio button two receives focus.
4. Press tab once. The text box ("You can even...") receives focus.

Steps to reproduce

1. Click "Radio 2" to select radio button number two.
2. Focus the number input field ("999").
3. Press enter once. Notice that radio button one receives focus. This is the bug.
4. Press enter once. The text box ("You can even...") receives focus.

Same thing goes for shift-tab and shift-enter.

Test cases

Needs test cases where, out of three radio buttons

• no radio button is selected.
• where the first is selected.
• where the middle is selected.
• where the last is selected.