Vendor prefixed is awol

These work:

```html
spec:css-2018; type:dfn; text:vendor-prefixed
spec:css-2020; type:dfn; text:vendor-prefixed
```

while this does not

```html
spec:css-2021; type:dfn; text:vendor-prefixed
```

It seems odd to have to link to the previous snapshot for the definition. Was this a deliberate change, or an accident?
