Steps:
I have written up the steps in the last comment here:
https://community.adobe.com/t5/acrobat/create-pdf-from-web-page-uses-only-light-weight-fonts/m-p/11945480
- make an HTML file from the provided code
- make a PDF from this HTML file
- see if your result is different from the attached PDF in the forum post

Environment:
Acrobat DC (CC) 21.0 on Mac OS X Big Sur 11.2.3
Helvetica Neue typeface installed with system: /System/Library/Fonts/HelveticaNeue.ttc

Expected:
A regular (or even medium) weight of a typeface would be chosen.

Observed:
The lightest available weight of the typeface is chosen, making the resulting PDF unreadable. PDF attached to forum post is also attached here.