`place-self` values: `center`, `flex-start`, `flex-end`

Upon checking my Grid layout's CSS, the validator reported the values `center`, `flex-start`, `flex-end` are invalid for the property `place-self`.

My CSS was:
```css
    .classA{ place-self: stretch center }
    .classB{ place-self: stretch flex-start }
    .classC{ place-self: stretch flex-end }
```