[About W3C standards] Is it necessary to strictly adhere to the DTD standards to which elements should be applied? _html/css_WEB-ITnose

Source of the problem:
In the process of learning, I found that HTML4.01, But during the practice, when I learned to write the surface layer of a website, I found that it can be used normally. For example, the

tag, html5, XHTML1.0 and HTML4.01 Strict are not supported, but After writing it, it can still be displayed in the center normally.


My question:
(1) http://www.w3school.com.cn/tags/html_ref_dtd.asp
Such a standard, here What does it mean to say "no" or "no"? What exactly does it mean? Does it mean unavailable? (Looks like not)
(2) Do the standards listed by W3C (such as the standard at the end of the link) must be strictly followed in actual projects? What problems will occur if it is violated? Give me an example to clarify. Thank you!

Reply to discussion (solution)

First of all, the information on this website is not completely accurate.

Secondly, H5 was released from 08 It has been continuously improved since then and was completed in 2014.
This final standard specification of W3C has no direct relationship with us ordinary developers
but is related to the product versions launched by major browser manufacturers.
We are working on these browsers again.
/**************The above is all nonsense*****************/

Question 1.NO is, new element, deleted element.
Question 2. Must follow strict /Mixed mode, it depends on the project, it is actually not that exaggerated.
Now the development project is directly

/**************The following is also nonsense*******************/
In actual development We rarely use NO tags.
tags like , are never used at all.
If you want to use some new familiar tags like