DOCTYPE is a necessary declaration for HTML documents that specifies the version of HTML used by the browser to parse web pages. 1. It is not an HTML tag, but a directive that ensures that the browser enters standard mode rather than weird mode. 2. Missing or incorrect DOCTYPE will cause rendering exceptions, affecting page layout and functionality. 3. It is recommended to use DOCTYPE of HTML5, that is, , which is concise and universal. 4. Must be placed at the top of the document, case-insensitive, and no need to close the label. Correct use of DOCTYPE can improve cross-browser compatibility, support modern web features, and avoid styling problems.
DOCTYPE seems inconspicuous in HTML, but it actually determines how the browser parses and renders web pages. Simply put, it is not an HTML tag, but a directive that tells the browser which HTML version to use for parsing. Without it, the browser may go into "weird mode", causing the page to display exceptions.

What is DOCTYPE?
The full name of DOCTYPE is Document Type Declaration. It appears at the top of the HTML document, telling the browser which HTML or XHTML specification the current document follows. For example, HTML5's DOCTYPE only needs to be written , which is concise and unified, not as complicated as previous HTML 4.01 or XHTML.
It does not perform any function itself, but its existence affects the rendering behavior of the browser. For example, if DOCTYPE is missing, the IE browser may use the old parsing method, resulting in a confusing style.

Why is DOCTYPE important?
- Avoid weird modes : The browser has two rendering modes, standard mode and weird mode. Missing or wrong DOCTYPE causes the browser to enter weird mode and rendering behavior becomes unpredictable.
- Ensure consistency : Different browsers perform more consistently in standard mode, which helps cross-browser compatibility.
- Support for modern features : Only in standard mode, new features of HTML5 and CSS3 can be correctly parsed and applied.
For example, if you are making a responsive web page but forget to add DOCTYPE, then the @media
query may not work and the layout will be messy.
How to use DOCTYPE correctly?
For modern web development, it is recommended to use HTML5's DOCTYPE because it is the simplest and most general:

<!DOCTYPE html>
A few things to note:
- It must be placed at the front of the HTML file, and there must be no content (including spaces or comments).
- It is case-insensitive, but is usually written in full capital.
- No closing tags are required, no extra parameters are required (not like the older version).
If you see some old websites using statements like , it means they are still using the old standards.
Ending
In general, DOCTYPE is one of the basic settings of HTML pages. Although it looks simple, its function cannot be ignored. Just add to allow the browser to enter standard mode and avoid many unnecessary compatibility issues. Basically that's it.
The above is the detailed content of The Importance of Doctype in HTML. For more information, please follow other related articles on the PHP Chinese website!

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