3. Doctype's document type

Doctype role? How to distinguish between strict mode and promiscuous mode? What's the point?

The declaration must be the first line of the HTML document, before the tag.

A declaration is not an HTML tag; it is an instruction to the web browser as to which version of HTML the page was written with.

Strict mode typography and JS work to the highest standard supported by the browser.

DOCTYPE not present or malformed will cause the document to render in promiscuous mode

In promiscuous mode, pages are displayed in a loose backwards compatible manner. Emulates the behavior of older browsers to prevent sites from not working.

Doctype document type This tag declares three DTD types, representing strict, transitional, and frame-based HTML documents.

HTML5 has only one<!DOCTYPE html>
HTML 4.01 specifies three document types: Strict, Transitional, and Frameset.
XHTML 1.0 specifies three XML document types: Strict, Transitional, and Frameset.
Standards mode (aka strict rendering mode) is used to render web pages that follow the latest standards, while Quirks
(Inclusive) mode (aka loose rendering mode or compatibility mode) is used to render web pages designed for legacy browsers.

Knowledge point: HTML commonly used DOCTYPE declaration

# HTML 5
<!DOCTYPE html>

# HTML 4.01 Strict
该 DTD 包含所有 HTML 元素和属性，但不包括展示性的和弃用的元素（比如 font）。不允许框架集（Framesets）。
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">

# HTML 4.01 Transitional
该 DTD 包含所有 HTML 元素和属性，包括展示性的和弃用的元素（比如 font）。不允许框架集（Framesets）。
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">

# HTML 4.01 Frameset
该 DTD 等同于 HTML 4.01 Transitional，但允许框架集内容。
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN"
"http://www.w3.org/TR/html4/frameset.dtd">

Knowledge point: DOCTYPE declaration commonly used in XHTML

# XHTML 1.0 Strict
该 DTD 包含所有 HTML 元素和属性，但不包括展示性的和弃用的元素（比如 font）。不允许框架集（Framesets）。必须以格式正确的 XML 来编写标记。
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

# XHTML 1.0 Transitional
该 DTD 包含所有 HTML 元素和属性，包括展示性的和弃用的元素（比如 font）。不允许框架集（Framesets）。必须以格式正确的 XML 来编写标记。
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "
http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

# XHTML 1.0 Frameset
该 DTD 等同于 XHTML 1.0 Transitional，但允许框架集内容.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">

# XHTML 1.1
该 DTD 等同于 XHTML 1.0 Strict，但允许添加模型（例如提供对东亚语系的 ruby 支持）。
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

3. Doctype's document type

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