Why are You teaching something that isn't in HTML5?

Question

Good day! In mnigih Your lessons have inscriptions about this content: «Тег <frameset> был отменен в языке HTML5».
Why are You teaching something that isn't in HTML5?

Answer 1

Not all sites written in html5 have old versions that have and support this tag.

Answer 2

Internet Explorer

Answer 3

It is in the html basics and not html5 section but you have a point.