Hyper Text Markup Language

Welcome to the I Can't Sleep podcast where I read random articles from across the web to bore you to sleep with my soothing voice.

I'm your host Benjamin Boster.

Today's episode is from a Wikipedia article titled HTML.

The hypertext markup language or HTML is the standard markup language for documents designed to be displayed in a web browser.

It defines the meaning and structure of web content.

It is often assisted by technologies such as cascading stylesheets,

CSS,

And scripting languages such as JavaScript.

Web browsers receive HTML documents from a web server or local storage and render the documents into multimedia web pages.

HTML describes the structure of a web page semantically and originally included cues for its appearance.

HTML elements are the building blocks of HTML pages.

With HTML constructs images and other objects such as interactive forms may be embedded into the rendered page.

HTML provides a means to create structured documents by denoting structural semantics for text such as headings,

Paragraphs,

Lists,

Links,

Quotes,

And other items.

HTML elements are delineated by tags written using angle brackets.

Tags such as IMG and input directly produce content into the page.

Other tags such as paragraph and closed paragraph surround and provide information about document text and may include sub element tags.

Browsers do not display the HTML tags but use them to interpret the content of the page.

HTML can embed programs written in a scripting language such as JavaScript which affects the behavior and content of web pages.

The inclusion of CSS defines the look and layout of content.

The World Wide Web Consortium,

W3C,

Former maintainer of the HTML and current maintainer of the CSS standards,

Has encouraged the use of CSS over explicit presentational HTML since 1997.

A form of HTML known as HTML5 is used to display video and audio primarily using the canvas element together with JavaScript.

In 1980 physicist Tim Berners-Lee,

A contractor at CERN,

Proposed and prototyped Inquire,

A system for CERN researchers to use and share documents.

In 1989 Berners-Lee wrote a memo proposing an internet-based hypertext system.

Berners-Lee specified HTML and wrote the browser and server software in late 1990.

That year Berners-Lee and CERN data systems engineer Robert Cahlia collaborated on a joint request for funding,

But the project was not formally adopted by CERN.

In his personal notes of 1990,

Berners-Lee listed some of the many areas in which hypertext used and Encyclopedia is the first entry.

The first publicly available description of HTML was a document called HTML Tags,

First mentioned on the Internet by Tim Berners-Lee in late 1991.

It describes 18 elements comprising the initial relatively simple design of HTML.

Except for hyperlink tag,

These were strongly influenced by SGML,

An in-house standard generalized markup language based documentation format at CERN.

11 of these elements still exist in HTML4.

HTML is a markup language that web browsers use to interpret and compose text,

Images,

And other material into visible or audible web pages.

Default characteristics for every item of HTML markup are defined in the browser,

And these characteristics can be altered or enhanced by the web page designers additional use of CSS.

Many of the text elements are mentioned in the 1988 ISO Technical Report,

TR 9537,

Techniques for Using SGML,

Which describes the features of early text formatting languages such as that used by the runoff command developed in the early 1960s for the CTSS compatible timesharing system operating system.

These formatting commands were derived from the commands used by typesetters to manually format documents.

However,

The SGML concept of generalized markup is based on elements,

Nested annotated ranges with attributes,

Rather than merely print effects with separate structure and markup.

HTML has been progressively moved in this direction with CSS.

Berners-Lee considered HTML to be an application of SGML.

It was formally defined as such by the Internet Engineering Task Force,

IETF,

With the mid-1993 publication of the first proposal for an HTML specification,

The Hypertext Markup Language Internet Draft by Berners-Lee and Dan Connolly,

Which included an SGML document type definition to define the syntax.

The draft expired after six months,

But was notable for its acknowledgement of the NCSA Mosaic browser's custom tag for embedding inline images,

Reflecting the IETF's philosophy of basing standards on successful prototypes.

Similarly,

Dave Raggett's competing internet draft HTML+,

Hypertext Markup Format,

From late 1993,

Suggested standardizing already implemented features like tables and fill-out forms.

After the HTML and HTML+,

Drafts expired in early 1994,

The IETF created an HTML working group.

In 1995,

This working group completed HTML 2.

0,

The first HTML specification intended to be treated as a standard against which future implementations should be based.

Further development under the auspices of the IETF was stalled by competing interests.

Since 1996,

The HTML specifications have been maintained,

With input from commercial software vendors by the World Wide Web Consortium.

In 2000,

HTML became an international standard.

HTML 4.

01 was published in late 1999,

With further errata published through 2001.

In 2004,

Development began on HTML 5 in the Web Hypertext Application Technology Working Group,

Which became a joint deliverable with the W3C in 2008,

And was completed and standardized on the 28th of October 2014.

HTML version timeline.

HTML 2.

November 24th,

1995.

HTML 2.

0 was published as RFC 1866.

Supplemental RFCs added capabilities.

November 25th,

1995.

RFC 1867,

Form-based file upload.

May 1996,

RFC 1942,

Tables.

August 1996,

RFC 1980,

Client-side image maps.

January 1997,

RFC 2070,

Internationalization.

HTML 3,

January 14th,

1997.

HTML 3.

2 was published as a W3C recommendation.

It was the first version developed and standardized exclusively by the W3C,

As the IETF had closed its HTML working group on September 12th,

1996.

Initially codenamed Wilbur,

HTML 3.

2 dropped math formulas entirely,

Reconciled overlap among various proprietary extensions,

And adopted most of Netscape's visual markup tags.

Netscape's bling element and Microsoft's marquee element were omitted due to a mutual agreement between the two companies.

A markup for mathematical formulas similar to that of HTML was standardized 14 months later in MathML.

HTML 4,

December 18th,

1997.

HTML 4 was published as a W3C recommendation.

It offers three variations.

Strict,

In which deprecated elements are forbidden.

Transitional,

In which deprecated elements are allowed.

Frameset,

In which mostly only frame-related elements are allowed.

Initially codenamed Cougar,

HTML 4.

0 adopted many browser-specific element types and attributes,

But also sought to phase out Netscape's visual markup features by marking them as deprecated in favor of style sheets.

HTML 4 is an SGML application conforming to ISO 8879-SGML.

April 24th,

1998.

HTML 4.

0 was reissued with minor edits,

Without incrementing the version number.

December 24th,

1999.

HTML 4.

01 was published as a W3C recommendation.

It offers the same three variations as HTML 4.

0,

And its last errata were published on May 12th,

2001.

May 2000.

ISO IEC 15445-2000.

ISO HTML based on HTML 4.

01 Strict was published as an ISO IEC international standard.

In the ISO,

This standard is in the domain of the ISO IEC JTC 1 SC 34.

After HTML 4.

01,

There were no new versions of HTML for many years,

As the development of the parallel XML-based language XHTML occupied the W3C's HTML working group.

HTML 5.

October 28th,

2014.

HTML 5 was published as a W3C recommendation.

November 1st,

2016.

HTML 5.

1 was published as a W3C recommendation.

December 14th,

2017.

HTML 5.

2 was published as a W3C recommendation.

HTML draft version timeline.

October 1991.

HTML tags.

An informal CERN document listing 18 HTML tags was first mentioned in public.

June 1992.

First informal draft of the HTML DTD with seven subsequent revisions.

July 15th,

August 6th,

August 18th,

November 17th,

November 19th,

November 20th,

November 22nd.

November 1992.

HTML DTD 1.

1.

The first with a version number based on RCS revisions,

Which start with 1.

1 rather than 1.

0.

An informal draft.

June 1993.

Hypertext markup language was published by the IETF IIIR working group as an internet draft.

A rough proposal for a standard.

It was replaced by a second version one month later.

November 1993.

HTML Plus was published by the IETF as an internet draft and was a competing proposal to the hypertext markup language draft.

It expired in July 1994.

November 1994.

First draft,

Version 0.

0 of HTML 2.

0 published by IETF itself,

Called as HTML 2.

0 from revision 02.

That finally led to the publication of RFC 1866 in November 1995.

April 1995.

Authored March 1995.

HTML 3.

0 was proposed as a standard for the IETF,

But the proposal expired five months later without further action.

It included many of the capabilities that were in Ragged's HTML Plus proposal,

Such as support for tables,

Text flow around figures,

And the display of complex mathematical formulas.

W3C began development of its own arena browser as a testbed for HTML 3 and cascading stylesheets,

But HTML 3.

0 did not succeed for several reasons.

The draft was considered very large at 150 pages,

And the pace of browser development,

As well as the number of interested parties,

Had outstripped the resources of the IETF.

Browser vendors,

Including Microsoft and Netscape at the time,

Chose to implement different subsets of HTML 3's draft features,

As well as to introduce their own extensions to it.

These included extensions to control stylistic aspects of documents,

Contrary to the belief of the academic engineering community that such things as text color,

Background texture,

Font size,

And font face were definitely outside the scope of a language when their only intent was to specify how a document would be organized.

Dave Raggett,

Who has been a W3C fellow for many years,

Has commented,

For example,

To a certain extent,

Microsoft built its business on the web by extending HTML features.

January 2008,

HTML 5 was published as a working draft by the W3C.

Although its syntax closely resembles that of SGML,

HTML 5 has abandoned any attempt to be an SGML application and has explicitly defined its own HTML serialization,

In addition to an alternative XML-based XHTML5 serialization.

2011 HTML 5 Last Call.

On the 14th of February 2011,

The W3C extended the charger of its HTML working group with clear milestones for HTML 5.

In May 2011,

The working group advanced HTML 5 to Last Call,

An invitation to communities inside and outside W3C to confirm the technical soundness of the specification.

The W3C developed a comprehensive test suite to achieve broad interoperability for the full specification by 2014,

Which was the target date for recommendation.

In January 2011,

The WHATWG renamed its HTML 5 living standard to HTML.

The W3C nevertheless continues its project to release HTML 5.

2012 HTML 5 Candidate Recommendation.

In July 2012,

WHATWG and W3C decided on a degree of separation.

W3C will continue with HTML 5 specification work,

Focusing on a single definitive standard,

Which is considering a snapshot by WHATWG.

The WHATWG organization will continue its work with HTML 5 as a living standard.

The concept of a living standard is that it is never complete and is always being updated and improved.

New features can be added,

But functionality will not be removed.

In December 2012,

W3C designated HTML 5 as a Candidate Recommendation.

The criterion for advancement to W3C recommendation is two 100% complete and fully interoperable implementations.

2014 HTML 5 Proposed Recommendation and Recommendation.

In September 2014,

W3C moved HTML 5 to Proposed Recommendation.

On 28th of October 2014,

HTML 5 was released as a stable W3C recommendation,

Meaning the specification process is complete.

XHTML is a separate language that began as a reformulation of HTML 4.

01 using XML 1.

0.

It is now referred to as the XML syntax for HTML and no longer being developed as a separate standard.

XHTML 1.

0 was published as a W3C recommendation on January 26,

2000 and was later revised and republished on August 1,

2002.

It offers the same three variations as HTML 4.

0 and 4.

01,

Reformulated in XML with minor restrictions.

XHTML 1.

1 was published as a W3C recommendation on May 31,

2001.

It is based on XHTML 1.

0 strict that includes minor changes,

Can be customized,

And is reformulated using modules in the W3C recommendation,

Modularization of XHTML,

Which was published on April 10,

2001.

XHTML 2.

0 was a working draft.

Work on it was abandoned in 2009 in favor of work on HTML 5 and XHTML 5.

XHTML 2.

0 was incompatible with XHTML 1.

X and therefore would be more accurately characterized as an XHTML inspired new language than an update to XHTML 1.

X.

On the 28th of May 2019,

The W3C announced that WHATWG would be the sole publisher of the HTML and DOM standards.

The W3C and the WHATWG had been publishing competing standards since 2012.

While the W3C standard was identical to the WHATWG in 2007,

The standards have since progressively diverged due to different design decisions.

The WHATWG living standard has been the de facto web standard for some time.

HTML markup consists of several key components including those called tags and their attributes,

Character-based data types,

Character references,

And entity references.

HTML tags most commonly come in pairs like H1 and end H1,

Although some represent empty elements and so are unpaired.

For example,

IMG.

The first tag in such a pair is the start tag and the second is the end tag.

They are also called opening tags and closing tags.

Another important component is the HTML document type declaration,

Which triggers standards mode rendering.

The following is an example of a classic Hello World program.

Doctype HTML,

HTML,

Head,

Title.

This is a title.

End title.

End head.

Body.

Div.

Paragraph.

Hello world.

End paragraph.

End div.

End body.

End HTML.

The text between HTML and end HTML describes the web page and the text between body and end body is the visible page content.

The markup text title,

This is a title,

End title,

Defines the browser page title shown on browser tabs and window titles,

And the tag div defines a division of the page used for easy styling.

Between head and end head,

A meta element can be used to define web page metadata.

The document type declaration,

Doctype HTML,

Is for HTML 5.

If a declaration is not included,

Various browsers will revert to quirks mode for rendering.

HTML documents imply a structure of nested HTML elements.

These are indicated in the document by HTML tags enclosed in angle brackets.

In the simple general case,

The extent of an element is indicated by a pair of tags,

A start tag,

Paragraph,

And end tag,

End paragraph.

The text content of the element,

If any,

Is placed between these tags.

Tags may also enclose further tag markup between the start and end,

Including a mixture of tags and text.

This indicates further nested elements as children of the parent element.

The start tag may also include the element's attributes within the tag.

These indicate other information,

Such as identifiers for sections within the document,

Identifiers used to bind style information to the presentation of the document,

And for some tags,

Such as the image used to embed images,

The reference to the image resource in the format,

Like this,

Img source equals example.

Com forward slash example dot jpg.

Some elements,

Such as the line break,

Do not permit any embedded content,

Either in text or further tags.

These require only a single empty tag akin to a start tag and do not use an end tag.

Many tags,

Particularly the closing end tag for the very commonly used paragraph element,

Are optional.

An HTML browser or other agent can infer the closure for the end of an element from the context and the structural rules defined by the HTML standard.

These rules are complex and not widely understood by most HTML authors.

The general form of an HTML element is therefore tag attribute 1 equals value 1,

Attribute 2 equals value 2,

Content tag.

Some HTML elements are defined as empty elements and take the form tag attribute 1 equals value 1,

Attribute 2 equals value 2.

Empty elements may enclose no content,

For instance,

The break tag or the inline image tag.

The name of an HTML element is the name used in the tags.

The end tag's name is preceded by a slash character,

And that in empty elements the end tag is neither required nor allowed.

If attributes are not mentioned,

Default values are used in each case.

Element examples.

Headings.

HTML headings are defined with the h1 to h6 tags with h1 being the highest or most important level and h6 the least.

CSS can drastically change the rendering.

Paragraphs.

Line breaks.

The difference between break and paragraph is that break breaks a line without altering the semantic structure of the page,

Whereas paragraph sections the page into paragraphs.

The element break is an empty element in that although it may have attributes,

It can take no content and it may not have an end tag.

Inputs.

There are many possible ways a user can give inputs,

Like input type equals text,

Input type equals file,

Input type equals checkbox.

Comments.

Comments can help in the understanding of the markup and do not display in the web page.

There are several types of markup elements used in HTML.

Structural markup indicates the purpose of text.

For example,

H2 golf close h2 establishes golf as a second level heading.

Structural markup does not denote any specific rendering,

But most web browsers have default styles for element formatting.

Content may be further styled using cascading style sheets CSS.

Presentational markup indicates the appearance of the text regardless of its purpose.

For example,

Bold text indicates that visual output devices should render boldface and bold text,

But gives a little indication what devices that are unable to do this,

Such as aural devices that read the text aloud,

Should do.

In the case of both bold text and italic text,

There are other elements that may have equivalent visual renderings,

But that are more semantic in nature,

Such as strong text and emphasized text,

Respectively.

It is easier to see how an aural user agent should interpret the latter two elements.

However,

They are not equivalent to their presentational counterparts.

It would be undesirable for a screen reader to emphasize the name of a book,

For instance,

But on a screen such a name would be italicized.

Most presentational markup elements have become deprecated under the HTML 4.

0 specification in favor of using CSS for styling.

Hypertext markup makes parts of a document into links to other documents.

An anchor element creates a hyperlink in the document,

And its href attribute sets the link's target URL.

For example,

The HTML markup a href equals HTTPS colon forward slash forward slash en dot Wikipedia dot org forward slash will render the word Wikipedia as a hyperlink.

To render an image as a hyperlink,

An image element is inserted as content into the a element.

Like break,

Image is an empty element with attributes,

But no content or closing tag.

A href equals HTTPS colon forward slash forward slash example dot org image source equals image dot gif.

Alt equals descriptive text,

Width equals 50,

Height equals 50,

Order equals zero.

Attributes.

Most of the attributes of an element are name value pairs,

Separated by equals,

And written within the start tag of an element after the element's name.

The value may be enclosed in single or double quotes,

Although values consisting of certain characters can be left unquoted in HTML,

But not XHTML.

Leaving attribute values unquoted is considered unsafe.

In contrast,

With name value pair attributes,

There are some attributes that affect the element simply by their presence in the start tag of the element,

Like the is map attribute for the image element.

There are several common attributes that may appear in many elements.

The ID attribute provides a document-wide unique identifier for an element.

This is used to identify the element so that style sheets can alter its presentational properties,

And scripts may alter,

Animate,

Or delete its content or presentation.

Appended to the URL of the page,

It provides a globally unique identifier for the element,

Typically a subsection of the page.

For example,

The ID attributes in HTTPS colon forward slash forward slash en dot wikipedia dot org forward slash wiki forward slash HTML attributes.

The class attribute provides a way of classifying similar elements.

This can be used for semantic or presentation purposes.

For example,

An HTML document might semantically use the designation class equals notation to indicate that all elements with this class value are subordinate to the main text of the document.

In presentation,

Such elements might be gathered together and presented as footnotes on a page instead of appearing in the place where they occur in the HTML source.

Class attributes are used semantically in microformats.

Multiple class values may be specified.

For example,

Class equals notation import puts the element into both the notation and the import classes.

An author may use the style attribute to assign presentational properties to a particular element.

It is considered better practice to use an element's ID or class attributes to select the element from within a style sheet,

Though sometimes this can be too cumbersome for a simple,

Specific,

Or ad hoc styling.

The title attribute is used to attach a subtextual explanation of an element.

In most browsers,

This attribute is displayed as a tooltip.

The lang attribute identifies the natural language of the element's contents,

Which may be different from that of the rest of the document.

For example,

In an English language document,

Paragraph OL span lang equals French,

C'est la vie,

Close span,

As they say in France,

Close paragraph.

The abbreviation element ABBR can be used to demonstrate some of these attributes.

Abbreviation ID equals NID,

Class equals jargon,

Style equals color purple,

Title equals hypertext markup language,

HTML,

Close abbreviation.

This example displays as HTML,

And most browsers pointing the cursor at the abbreviation should display the title text,

Hypertext markup language.

Most elements take the language-related attribute DIR to specify text direction,

Such as with RTL for right-to-left text in,

For example,

Arabic,

Persian,

Or Hebrew.

Character and entity references.

As of version 4.

0,

HTML defines a set of 252 character entity references,

And a set of 1,

114,

050 numeric character references,

Both of which allow individual characters to be written via simple markup,

Rather than literally.

A literal character and its markup counterpart are considered equivalent and are rendered identically.

The ability to escape characters in this way allows for the characters less than and ampersand,

When written as ampersand LT,

Semicolon,

And ampersand AMP,

Semicolon,

Respectively,

To be interpreted as character data,

Rather than markup.

For example,

A literal less than normally indicates the start of a tag,

And ampersand normally indicates the start of a character entity reference,

Or numeric character reference.

Writing it as ampersand AMP,

Semicolon,

Or ampersand hashtag x26,

Semicolon,

Or ampersand hashtag 38,

Semicolon,

Allows ampersand to be included in the content of an element or in the value of an attribute.

The double quote character,

When not used to quote an attribute value,

Must also be escaped as ampersand quote,

Semicolon,

Or ampersand hashtag x22,

Semicolon,

Or ampersand hashtag 34,

Semicolon,

When it appears within the attribute value itself.

Equivalently,

The single quote character,

When not used to quote an attribute value,

Must also be escaped as ampersand hashtag x27,

Semicolon,

Or ampersand hashtag 39,

Semicolon,

Or as ampersand APOS,

Semicolon,

In HTML5 or XHTML5 documents,

When it appears within the attribute value itself.

If document authors overlook the need to escape such characters,

Some browsers can be very forgiving and try to use context to guess their intent.

The result is still invalid markup,

Which makes the document less accessible to other browsers and other user agents that may try to parse the document for search and indexing purposes,

For example.

Escaping also allows for characters that are not easily typed or that are not available in the document's character encoding to be represented within the element and attribute content.

For example,

The acute accented E,

A character typically found only on Western European and South American keyboards,

Can be written in any HTML document as the entity reference ampersand E acute,

Semicolon,

Or as the numeric reference ampersand hashtag xE9,

Semicolon,

Or ampersand hashtag 233,

Semicolon,

Using characters that are available on all keyboards and are supported in all character encodings.

Unicode character encodings,

Such as UTF-8,

Are compatible with all modern browsers and allow direct access to almost all the characters of the world's writing systems.

Data types.

HTML defines several data types for element content,

Such as script data and style sheet data,

And a plethora of types for attribute values,

Including IDs,

Names,

URIs,

Numbers,

Units of length,

Languages,

Media descriptors,

Colors,

Character encodings,

Dates and times,

And so on.

All of these data types are specializations of character data.

Document type declaration.

HTML documents are required to start with a document type declaration,

Informally a DOCTYPE.

In browsers,

The DOCTYPE helps to define the rendering mode,

Particularly whether to use Quirks mode.

The original purpose of the DOCTYPE was to enable the parsing and validation of HTML documents by HTML tools based on the document type definition,

DTD.

The DTD to which the DOCTYPE refers contains a machine-readable grammar specifying the permitted and prohibited content for a document,

Conforming to such a DTD.

Browsers,

On the other hand,

Do not implement HTML as an application of SGML,

And as consequence,

Do not read the DTD.

HTML 5 does not define a DTD.

Therefore,

In HTML 5,

The DOCTYPE declaration is simpler and shorter.

DOCTYPE HTML.

For example,

An HTML 4 DOCTYPE.

DOCTYPE html-public-forward-slash-forward-slash-w3c-forward-slash-dtd-html-4.

01-forward-slash-en-https-colon-forward-slash-www.

W3.

Org-forward-slash-tr-forward-slash-html4-forward-slash-strict.

Dtd-html-public-html-public-html-public-html-public-html-public-html-public-html-public-html-public-html-public-html-public-html-public-html-public-html-public-html-public-html-public-html-public-html-public-html-public-html-public-html-p This declaration references the DTD for the strict version of HTML 4.

01.

HTML-based validators read the DTD in order to properly parse the document,

And to perform validation.

In modern browsers,

A valid DOCTYPE activates STANDARDS mode as opposed to QUIRKS mode.

In addition,

HTML 4.

01 provides transitional and frameset DTDs as explained below.

The transitional type is the most inclusive,

Incorporating current tags as well as older or deprecated tags,

With the strict DTD excluding deprecated tags.

The frameset has all tags necessary to make frames on a page along with the tags included in transitional type.

Semantic HTML is a way of writing HTML that emphasizes the meaning of the encoded information over its presentation or look.

HTML has included semantic markup from its inception,

But has also included presentational markup such as font,

Italics,

And center tags.

There are also the semantical,

Neutral,

Div,

And span tags.

Since the late 1990s,

When cascading stylesheets were beginning to work in most browsers,

Web authors have been encouraged to avoid the use of presentational HTML markup with a view to the separation of content and presentation.

In a 2001 discussion of the Semantic Web,

Tim Berners-Lee and others gave examples of ways in which intelligent software agents may one day automatically crawl the web and find,

Filter,

And correlate previously unrelated published facts for the benefit of human users.

Such agents are not commonplace even now,

But some of the ideas of Web 2.

0,

Mashups,

And price comparison websites may be coming close.

The main difference between these web application hybrids and Berners-Lee's semantic agents lies in the fact that the current aggregation and hybridization of information is usually designed by web developers,

Who already know the web locations and the API semantics of the specific data they wish to mash,

Compare,

And combine.

An important type of web agent that does crawl and read web pages automatically without prior knowledge of what it might find is the web crawler or search engine spider.

These software agents are dependent on the semantic clarity of web pages they find as they use various techniques and algorithms to read and index millions of web pages a day and provide web users with search facilities without which the World Wide Web's usefulness would be greatly reduced.

In order for search engine spiders to be able to rate the significance of pieces of text they find in HTML documents,

And also for those creating mashups and other hybrids as well as for more automated agents as they are developed,

The semantic structures that exist in HTML need to be widely and uniformly applied to bring out the meaning of the published text.

Presentational markup tags are deprecated in current HTML and XHTML recommendations.

A majority of presentational features from previous versions of HTML are no longer allowed,

As they lead to poor accessibility,

Higher cost of site maintenance,

And larger document sizes.

Good semantic HTML also improves the accessibility of web documents.

For example,

When a screen reader or audio browser can correctly ascertain the structure of a document,

It will not waste the visually impaired user's time by reading out repeated or irrelevant information when it has been marked up correctly.

HTML documents can be delivered by the same means as any other computer file.

However,

They are most often delivered either by HTTP from a web server or by email.

The World Wide Web is composed primarily of HTML documents transmitted from web servers to web browsers using the Hypertext Transfer Protocol,

HTTP.

However,

HTTP is used to serve images,

Sound,

And other content in addition to HTML.

To allow the web browser to know how to handle each document it receives,

Other information is transmitted along with the document.

This metadata usually includes the MIME type,

E.

G.

,

Text forward slash HTML or application forward slash XHTML plus XML,

And the character encoding.

In modern browsers,

The MIME type that is sent with the HTML document may affect how the document is initially interpreted.

A document sent with the XHTML MIME type is expected to be well-formed XML.

Syntax errors may cause the browser to fail to render it.

The same document sent with the HTML MIME type might be displayed successfully since some browsers are more lenient with HTML.

The W3C recommendations state that XHTML 1.

0 documents that follow guidelines set forth in the recommendations Appendix C may be labeled with either MIME type.

XHTML 1.

1 also states that XHTML 1.

1 documents should be labeled with either MIME type.

Most graphical email clients allow the use of a subset of HTML,

Often ill-defined,

To provide formatting and semantic markup not available with plain text.

This may include typographic information like colored headings,

Emphasized in quoted text,

Inline images,

And diagrams.

Many such clients include both a GUI editor for composing HTML email messages and a rendering engine for displaying them.

Use of HTML in email is criticized by some because of compatibility issues,

Because it can help disguise phishing attacks,

Because of accessibility issues for blind or visually impaired people,

Because it can consume spam filters,

And because the message size is larger than plain text.

The most common filename extension for files containing HTML is .

Html.

A common abbreviation of this is .

Htm,

Which originated because some early operating systems and file systems such as DOS and the limitations imposed by FAT data structure limited file extensions to three letters.

An HTML application,

Htafileextension.

Hta,

Is a Microsoft Windows application that uses HTML and dynamic HTML in a browser to provide the application's graphical interface.

A regular HTML file is confined to the security model of the web browser's security,

Communicating only to web servers and manipulating only web page objects and site cookies.

An HTA runs as a fully trusted application and therefore has more privileges,

Like creation,

Editing,

Removal of files,

And Windows registry entries.

Because they operate outside the browser's security model,

HTAs can be executed via HTTP,

But must be downloaded,

Just like an exe file,

And executed from local file system.

Since its inception,

HTML and its associated protocols gained acceptance relatively quickly.

However,

No clear standards existed in the early years of the language.

Though its creators originally conceived of HTML as a semantic language devoid of presentation details,

Practical uses pushed many presentational elements and attributes into the language,

Driven largely by the various browser vendors.

The latest standards surrounding HTML reflect efforts to overcome the sometimes chaotic development of the language,

And to create a rational foundation for building both meaningful and well-presented documents.

To return HTML to its role as a semantic language,

The W3C has developed style languages such as CSS and XSL to shoulder the burden of presentation.

In conjunction,

The HTML specification has slowly reigned in the presentational elements.

There are two axes differentiating various variations of HTML as currently specified.

SGML-based HTML versus XML-based HTML,

Referred to as XHTML,

On one axis,

And strict versus transitional,

Loose versus frameset,

On the other axis.