Why <iframe> isn't supported in html5?

Question

I was thinking in make a book inside my page... but what should i use to change the page's? often i use <iframe> .

Answer 1

It is not supported by the W3C HTML5 specification. Specifies that Document objects in the <iframe> element's browsing context are to be allowed to use requestFullscreen() (if it's not blocked for other reasons, e.g. there is another ancestor <iframe> without this attribute set).
https://www.quackit.com/html_5/tags/html_iframe_tag.cfm
https://www.w3schools.com/tags/tag_iframe.asp