Element retrieved in tagname becomes BODY instead of SELECT

I got b for the pull-down (select tag), so I want it to be b=SELECT, but for some reason it might be b=BODY.

When I checked the activeElement specifications on the page below, I found the following:

Document.activeElement-Web API|MDN

The focus on which element depends on the current configuration of the platform and browser.For example, macOS systems typically do not focus on anything other than text input elements by default.

I don't use macOS, and b=BODY occurs in Windows and Chrome environments. Does anyone know why?

vara= document.activeElement;
varb = a.tagName.toUpperCase();