| PASS cellIndex should exist. | |
| PASS For cells without a parent, cellIndex should be -1. | |
| PASS For cells whose parent is not a tr, cellIndex should be -1. | |
| PASS For cells whose parent is not a HTML tr, cellIndex should be -1. | |
| PASS For cells whose parent is a tr, cellIndex should be the index. | |
| PASS For cells whose parent is a tr with non td/th sibling, cellIndex should skip those non td/th siblings. | |