Section 4: Conformance: requirements and recommendations

--- tags: HTML4 SPEC --- # Section 4: Conformance: requirements and recommendations [link](https://www.w3.org/TR/html401/conform.html) ## Table of contents - [Purpose](#Purpose) - [Definitions](#Definitions) - [SGML](#SGML) - [The text/html content type](#The-texthtml-content-type) ## Purpose Begin the specification of HTML 4, starting with the contract between: - authors - documents - users - user agents ## Definitions - **HTML documents** - an SGML document / application - **Author** - a person writes HTML documents - a program generates HTML documents - **HTML user agent** - any device that interprets HTML documents - visual browsers (text-only and graphical) - non-visual browsers (audio, Braille) ![](https://i.imgur.com/aSDx8g0.png =350x300) - conforming user agent - A user agent should avoid imposing arbitrary length limits on attribute value - A user agent must ensure that rendering is unchanged by the presence or absence of start tags and end tags when the HTML DTD indicates that these are optional - For reasons of backwards compatibility, we recommend that tools interpreting HTML 4 continue to support HTML 3.2 and HTML 2.0 - **User** - a person who interacts with a user agent to view, hear, or otherwise use a rendered HTML document - **Error conditions** - This specification does not define how conforming user agents handle general error conditions - recommended error handling behavior: - cannot recognize **element**: render the element's content - cannot recognize **attribute**: ignore the entire attribute - cannot recognize **attribute value**: use the default attribute value - encounters an **undeclared entity**: treated as character data ([Recap: HTML Entity](https://www.w3schools.com/html/html_entities.asp)) - **Deprecated** - Specification clearly indicate which are deprecated - Deprecated elements may become **obsolete** in future versions of HTML - User agents should continue to support deprecated elements - Example: - HTML presentational attributes have been deprecated when **style sheet** alternatives - **Obsolete** - obsolete element or attribute is one for which there is **no guarantee** of support by a user agent ## SGML :::info HTML 4 is an SGML application ::: - User agents must not render SGML processing instructions (e.g., \<?full volume\>) or comments (Refer to [SGML limited support](https://www.w3.org/TR/html401/appendix/notes.html#sgmlfeatures))   :::info HTML 4.01 specifies **three DTDs**, so authors must include one of the following **document type declarations** in their documents ::: - [HTML 4.01 Strict DTD](https://www.w3.org/TR/html401/sgml/dtd.html) - includes all elements and attributes that have not been deprecated or do not appear in frameset documents - `<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">` - [HTML 4.01 Transitional DTD](https://www.w3.org/TR/html401/sgml/loosedtd.html) - includes everything in the strict DTD plus deprecated elements and attributes (most of which concern visual presentation) - `<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">` - [HTML 4.01 Frameset DTD](https://www.w3.org/TR/html401/sgml/framesetdtd.html) - includes everything in the transitional DTD plus frames as well - `<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">` - **Compare: Strict DTD vs Transitional DTD** <pre> =================== Headings ========================================= There are six levels of headings from H1 (the most important) to H6 (the least important). <!ELEMENT (%heading;) - - (%inline;)* -- heading --> <!ATTLIST (%heading;) %attrs; -- %coreattrs, %i18n, %events -- > </pre> <pre> =================== Headings ========================================= There are six levels of headings from H1 (the most important) to H6 (the least important). <!ELEMENT (%heading;) - - (%inline;)* -- heading --> <!ATTLIST (%heading;) %attrs; -- %coreattrs, %i18n, %events -- %align; -- align, text alignment -- > </pre> - **Compare: Transitional DTD vs Frameset DTD** <pre> <!ENTITY % HTML.Version "-//W3C//DTD HTML 4.01 Transitional//EN" -- Typical usage: <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> ... </head> <body> ... </body> </html> </pre> <pre> <!ENTITY % HTML.Version "-//W3C//DTD HTML 4.01 Frameset//EN" -- Typical usage: <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd"> <html> <head> ... </head> <frameset> ... </frameset> </html> </pre> ## The text/html content type HTML documents are sent over the Internet as a sequence of bytes accompanied by encoding information. A HTTP-message is either a request or a response: ``` HTTP-message = Request | Response ``` A HTTP-message has zero or more message-header⁠s and may have a message-body: ``` generic-message = start-line *(message-header CRLF) CRLF [ message-body ] ``` So not every HTTP-message has a message-body. But if it has a message-body, then that’s also the entity-body: ``` message-body = entity-body | <entity-body encoded as per Transfer-Encoding> ``` Entity Body (Request): ``` POST /foo HTTP/1.1 # Not part of the entity. Content-Type: text/plain # ┬ The entity is from this line down... Content-Length: 1234 # │ # │ Hello, World! ... # ┘ ``` Entity Body (Response): ``` HTTP/1.1 200 OK # Not part of the entity. Content-Length: 438 # ┬ The entity is from this line down... Content-Type: text/plain # │ # │ Response body ... # ┘ ``` :::info A message entity with a content type of "text/html" represents an HTML document. ::: The **content type** for HTML documents is defined as follows: - **Content type name**: text - **Content subtype name**: html - **Required parameters**: none - **Optional parameters**: charset - **Encoding considerations**: any encoding is allowed - **Security considerations**: See the notes on security   :::info How to specify this information ? ::: ```php <?php header('Content-Type: text/html; charset=utf-8'); ... ?> ``` To address server or configuration limitations, HTML documents may include explicit information about the document's character encoding; the META element can be used to provide user agents with this information. ```htmlembedded <META http-equiv="Content-Type" content="text/html; charset=EUC-JP"> ```

Syntax	Example	Reference
# Header	Header	基本排版
- Unordered List	Unordered List
1. Ordered List	Ordered List
- [ ] Todo List	Todo List
> Blockquote	Blockquote
Bold font	Bold font
Italics font	Italics font
~~Strikethrough~~	~~Strikethrough~~
19^th^	19^th
H~2~O	H₂O
++Inserted text++	Inserted text
==Marked text==	Marked text
[link text](https:// "title")	Link
![image alt](https:// "title")	Image
`Code`	`Code`	在筆記中貼入程式碼
```javascript var i = 0; ```	`var i = 0;`
:smile:		Emoji list
{%youtube youtube_id %}	Externals
$L^aT_eX$	L^aT_eX
:::info This is a alert area. :::	This is a alert area.