[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

,

.
In short, if there are no special requirements, on the basis of developing with XHTML, stick to HTML5.

Reply to webyellow: (1) Reply to the previous nonsense: In other words, these standards promoted by W3C (such as the link) are Provided to browser manufacturers as an adoptable specification when making browsers? As developers, we should understand the specifications of each browser rather than these, right?

(2) Reply to the next nonsense paragraph: (off-topic) I am still a beginner and have no work experience. After learning HTML CSS After that, let’s go into JS learning and then learn H5 C3, right? Or learn JS later? At present, I have decided to choose the former one. What are the advantages and disadvantages of the two learning paths? (I found that H5 is basically a variety of functional tags used with JS. It will be confusing to learn without JS)

(3) Reply to stuffing: How do you define and understand " Strict/mixed mode?
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I have enough problems and need to keep learning!

It is best not to use it because it may be abandoned. It is recommended to follow the standard, although the effect can be achieved.

1. There is no so-called standard in browsers, only support for the standard.

2. Personally, I don’t think it is necessary. Just learn whatever you like and what you think you need. Instead of dictating what you should learn, it is better to study some small things.
For example, what happens when a block element uses float? Wait
Life is very tolerant of mistakes, just follow your feelings.
3. What should I do now? It is to follow the leadership's arrangements.