Test vw-based root em font-size

In the visual design team, we've been discussing a simpler approach to responsive typography, which could do away with changing heading settings at 2 breakpoints as we currently do.

In my inbrowser experiments, defining the root font size on the html element like so:
```font-size: calc(.8rem + .2vw);``` seems to keep alignment to the baseline grid, and nicely scales all text while maintaining baseline alignment.

Wonder if we can test this in a pr? cc @jmuzina (very low priority).


Start by locating the current heading settings and their two responsive breakpoints in the framework styles. Compare them with a root html font-size of calc(.8rem + .2vw) using the existing visual examples or browser experiments. Done means determining whether the proposed scaling preserves baseline alignment and warrants a pull request.

Written by the indexing model from the issue text.