What do you think of adding a comment and a little content, then showing an example of the content at 1.x, 2x, and 3.5x? This will encourage others to test large scales.

<meta name="viewport" content="width=device-width">
<meta name="text-scale" content="scale">
<style>
h1 {
  /*
    Multiply the 2em baseline by a dampening factor (between 1.0 to 0.5) to
    ensure headers scale up, but without getting too large. Here are the
    resulting font sizes at various text scale factors:
            h1      body
      1x    32px    16px
      1.5x  40px    24px
      2x    48px    32px
      3x    64px    48px
      3.5x  72px    56px
  */
  --dampen-factor: calc(0.5 + 0.5 / env(preferred-text-scale));
  font-size: calc(2em * var(--dampen-factor));
}
</style>
<h1>Header</h1>
This is some body text. The header grows slower than the body text.

Yeah, that seems good, as long as @fantasai thinks it's appropriate level of detail for the spec. (We could put it in MDN or some other article/documentation.)

Also, note that you can see this PR at https://davidsgrogan.github.io/css-fonts-5/Overview.html#scale-keyword

Might be better to say, "Assuming a page hasn't altered the :root font size with an absolute length unit like px"

Bikeshed has put this paragraph in a separate note box, but this seems like a continuation of the previous one. Do they need to be in the same note box?

I think this intro needs more context. How about this:

If an element such as a heading already has large text, it is likely already large enough for a user who needs a high text scale to read. If the large text scales at the same rate as the body text, it could become too large to comfortably fit in the user's viewport.

Therefore, authors should consider using a dampening factor to reduce the rate that large text scales.

Might be good to use parentheses around the division so that people don't get confused by BIDMAS/PEMDAS. (Because I did. 😅 I was wondering for ages why it's not just 1 / env(preferred-text-scale).)

I think this needs stronger wording so that authors know 'you must only use this if you have tested your website with scaled text'.