Base font size and paragraph max-width

It would be nice for the spec to provide for…

1. base font-size
2. heading and paragraph max-width tokens. I usually define them for headings, body text and lead text. 

Maybe these could be included under typography, like the following?

``` yaml
typography:
  base: 16px
  measure:
    heading: 28rem
    lead: 36rem
    body: 38rem
```

Or, maybe somewhere else?