Document use cases and caveats of clamp()

[davelab6]

Now maybe not the best time to include this, but CSS clamp() seems relevant to using VFs.

Eg https://css-tricks.com/how-do-you-do-max-font-size-in-css/

[davelab6]

https://twitter.com/ScottKellum/status/1226878366059417600?s=19