I guess that XHTML is not another version of HTML. It's a convention to write better HTML. The only difference is that you have to follow a strict rule to write XHTML codes.
For 2nd question for recommendation is to use px. % will make the text size relative to screen (bigger screen bigger test and vice-versa). No idea about vw.
If you want to add responsiveness use media queries instead.of using relative values
Seniru Pasan yes. I was use media queries and px for size of the text.
A question: some useful tags example iframe is not in html. What tag can we replace for it?
FirstSoloKiller iframe is still supported in html
https://www.w3schools.com/tags/tag_iframe.asp
Only the frame element was deprecated.
NOTE: Some browsers are not implementing iframe feature
Stock to html5 as it is the most recent. px are what I use most of the time. However, there are times when I need to create large divs in relation to my screen. Here I find the % handy
