here in the picture below 1.6 rem = 16px;
but in my customized tribute page, there 1rem = 16px. (inspecting on p showes the font size 16px). why does it look different?
The r in rem is for root. So 1.6rem is 1.6× the size of whatever the root is. 1rem is 1× the size of whatever the root is. What’s the font size on the root element (the html tag)?
Spoiler:
First one is 10px, second one is 16px.
i didn’t set any font size for html tag
The default font - size in browser is 16px. The default size of 1em is 16px. ( font - size : 16px;).
While em is relative to the font-size of its direct or nearest parent, rem is only relative to the html (root) font-size.
and there is a lot more default settings for html 
google them up!
thank u so much Dorfieeee