The reason why you have to write is to prevent the browser from entering weird mode and ensure that the page is rendered correctly in standard mode. Specifically, it includes: 1. The browser can correctly identify HTML5 document types; 2. Avoid inconsistencies in box model calculation, default style and CSS characteristics due to weird patterns; 3. It is part of the HTML5 standard and is simpler, without reference to DTD; 4. In actual development, it is necessary to pay attention to its location, case and correct insertion during server rendering; 5. Most frameworks will handle it automatically, but it is still necessary to check whether the doctype exists in manual writing or in specific scenarios.
In modern web development, although the function of is simple, it is essential. It tells the browser that the current document is using HTML5 standards, ensuring that the page is rendered in standard mode, rather than compatibility mode. Although this step is inconspicuous, it directly affects the display effect and behavioral performance of the page.

Why do you have to write ?
Although most modern browsers can automatically recognize document types, not writing may cause the browser to enter "Quirks Mode". In this mode, the CSS parsing method will be closer to the behavior of early browsers, resulting in inconsistent layout and style.
Common phenomena include:

- Different calculation methods of box models
- The default margin and padding are inconsistent
- Some CSS features cannot be used properly
Therefore, to ensure that the page renders correctly in all browsers, it is recommended to always write at the beginning of the HTML document.
Its relationship with HTML5
is a standard document declaration method introduced by HTML5. Compared with previous XHTML or HTML 4.01, HTML5's doctype is much simpler and no longer requires reference to DTD (document type definition). This reduces the learning costs for developers and is easier to maintain.

Older version of doctype example (HTML 4.01):
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
The simplicity of HTML5 makes it more suitable for the needs of modern web development and is easier to remember and use.
Things to note in actual development
In actual development, there are several points that are easily overlooked:
- The location must be correct :
must be the first line of the HTML file, and there must be no whitespace characters or comments in front of it.
- Case insensitive : HTML is case-insensitive, so it is no problem to write
as <code> .
- Pay attention to server rendering : If you use React, Vue and other frameworks for server rendering, make sure that doctype is inserted correctly at the front of the generated HTML string.
If you use template engines (such as Pug, EJS), some will automatically insert doctype for you, but it may also require manual configuration.
Are tools and frameworks automatically processed?
Most modern front-end frameworks (such as Next.js, Gatsby, Vue CLI) will automatically add when generating HTML, so you may not need to add it manually. However, special attention should be paid to the following situations:
- When using static HTML templates
- When writing email templates or micro front-end pages manually
- When doing server-side rendering and splicing HTML strings
At this time, it is best to check the final output HTML source code to ensure that the doctype exists correctly.
Basically that's it. Although it is just a simple statement, it is the most basic and least overlooked part of modern web development.
The above is the detailed content of Using HTML `doctype` in Modern Web Development. For more information, please follow other related articles on the PHP Chinese website!

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