Thanks, Paul.
Now I think I understand what Helix is doing when it offers options to set font sizes in px, em, rem and percentage.
When you said that Helix uses Bootstrap 5, I realized that because Bootstrap 5 uses a default font-size 16px by default, then 16px becomes the root size. So no other size can be the root, such as assigning the H1 desktop size to 48 pixels. Right?
I had been under the impression that if I set a pixel size for an H1 tag in Helix in the desktop view, then that number would become the root number for the H1 tag. That might sound silly but it was how I was looking at settings Helix was offering.
But, what is the parent for an em setting in Helix?