I was working with Bootstrap 3.3.7 to learn it. I saw Bootstrap 4 was now available, so I downloaded it. However, the first thing I noticed was the "index.html" does not have either a "head" or "body" tag in it. Are we supposed to add them? Or, does Bootstrap 4 process HTML differently? I've never seen an "index.html" without those two tags. Why are they missing in Bootstrap 4?