Selected Courses on Digital Art-UOWM

2 Μαΐου 2013

b&h-pro-webd-tables-html-consuming-consumed-archive/ from simple html and js to dhtml , to html 5 and js sem 5-6

Filed under: Notes — admin @ 20:46
simple html
opening-closing concept
head section
body section
………..listen to the tutorial

uowm ftp server //

In more length: The term “HTML5” is widely used as a buzzword to refer to modern Web technologies, many of which (though by no means all) are developed at the WHATWG, in some cases in conjunction with the W3C and IETF.
The WHATWG work is all published in one specification (the one you are reading right now), parts of which are republished in an edition optimized for Web developers
The W3C also publishes parts of this specification as separate documents. One of these parts is called “HTML5”; it is a forked subset of this specification (the HTML Living Standard). There are numerous differences between this specification (the HTML Living Standard) and the W3C version, some minor, some major. Unfortunately these are not currently accurately documented anywhere, so there is no way to know which are intentional and which are not.


This section


This specification defines an abstract language for describing documents and applications, and some APIs for interacting with in-memory representations of resources that use this language.
The in-memory representation is known as “DOM HTML”, or “the DOM” for short.
There are various concrete syntaxes that can be used to transmit resources that use this abstract language, two of which are defined in this specification.
The first such concrete syntax is the HTML syntax. This is the format suggested for most authors. It is compatible with most legacy Web browsers. If a document is transmitted with the text/html MIME type, then it will be processed as an HTML document by Web browsers. This specification defines the latest HTML syntax, known simply as “HTML”.
The second concrete syntax is the XHTML syntax, which is an application of XML. When a document is transmitted with an XML MIME type, such as application/xhtml+xml, then it is treated as an XML document by Web browsers, to be parsed by an XML processor. Authors are reminded that the processing for XML and HTML differs; in particular, even minor syntax errors will prevent a document labeled as XML from being rendered fully, whereas they would be ignored in the HTML syntax. This specification defines the latest XHTML syntax, known simply as “XHTML”.
The DOM, the HTML syntax, and the XHTML syntax cannot all represent the same content. For example, namespaces cannot be represented using the HTML syntax, but they are supported in the DOM and in the XHTML syntax. Similarly, documents that use the noscript feature can be represented using the HTML syntax, but cannot be represented with the DOM or in the XHTML syntax. Comments that contain the string “-->” can only be represented in the DOM, not in the HTML and XHTML syntaxes.

αρχικό παράδειγμα γραφής dom html

A document with a short head

Here is an example of a longer one:

An application with a long head




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the spectacle and the image- the origins of spectacle

The theory of mass culture-or mass audience culture, commercial culture, “popular”
culture, the culture industry, as it is variously known-has always tended to define its object
against so-called high culture without reflecting on the objective status of this opposition.
As so often, positions in this field reduce themselves to two mirror-images, and are
essentially staged in terms of value. Thus the familiar motif of elitism argues for the priority
of mass culture on the grounds of the sheer numbers of people exposed to it; the pursuit of
high or hermetic culture is then stigmatized as a status hobby of small groups of
intellectuals. As its anti-intellectual thrust suggests, this essentially negative position has
little theoretical content but clearly responds to a deeply rooted conviction in American
radicalism and articulates a widely based sense that high culture is an establishment
phenomenon, irredeemably tainted by its association with institutions, in particular with
the university. The value invoked is therefore a social one: it would be preferable to deal
with tv programs, The Godfather, orJaws, rather than with Wallace Stevens or HenryJames,
because the former clearly speak a cultural language meaningful to far wider strata of the
population than what is socially represented by intellectuals. Radicals are however also
intellectuals, so that this position has suspicious overtones of the guilt trip; meanwhile it
overlooks the anti-social and critical, negative (although generally not revolutionary)
stance of much of the most important forms of modem art; finally, it offers no method for
reading even those cultural objects it valorizes and has had little of interest to say about
their content


Presentation related attributes

  • background (Deprecated. use CSS instead.) and bgcolor (Deprecated. use CSS instead.) attributes for body (required element according to the W3C.) element.
  • align (Deprecated. use CSS instead.) attribute on divform, paragraph (p) and heading (h1h6) elements
  • align (Deprecated. use CSS instead.), noshade (Deprecated. use CSS instead.), size (Deprecated. use CSS instead.) and width (Deprecated. use CSS instead.) attributes on hr element
  • align (Deprecated. use CSS instead.), bordervspace and hspace attributes on img and object (caution: the object element is only supported in Internet Explorer (from the major browsers)) elements
  • align (Deprecated. use CSS instead.) attribute on legend and caption elements
  • align (Deprecated. use CSS instead.) and bgcolor (Deprecated. use CSS instead.) on table element
  • nowrap (Obsolete), bgcolor (Deprecated. use CSS instead.), widthheight on td and th elements
  • bgcolor (Deprecated. use CSS instead.) attribute on tr element
  • clear (Obsolete) attribute on br element
  • compact attribute on dldir and menu elements
  • type (Deprecated. use CSS instead.), compact (Deprecated. use CSS instead.) and start (Deprecated. use CSS instead.) attributes on ol and ulelements
  • type and value attributes on li element
  • width attribute on pre element

HTML is written in the form of HTML elements consisting of tags enclosed in angle brackets (like ), within the web page content. HTML tags most commonly come in pairs like 


, although some tags, known as empty elements, are unpaired, for example . The first tag in a pair is the start tag, the second tag is the end tag (they are also called opening tags and closing tags). In between these tags web designers can add text, tags, comments and other types of text-based content.

Sample page

Sample page

This is a simple sample.

HTML documents 

Some features of HTML trade user convenience for a measure of user privacy.
In general, due to the Internet’s architecture, a user can be distinguished from another by the user’s IP address. IP addresses do not perfectly match to a user; as a user moves from device to device, or from network to network, their IP address will change; similarly, NAT routing, proxy servers, and shared computers enable packets that appear to all come from a single IP address to actually map to multiple users. Technologies such as onion routing can be used to further anonymize requests so that requests from a single user at one node on the Internet appear to come from many disparate parts of the network.
However, the IP address used for a user’s requests is not the only mechanism by which a user’s requests could be related to each other. Cookies, for example, are designed specifically to enable this, and are the basis of most of the Web’s session features that enable you to log into a site with which you have an account.
There are other mechanisms that are more subtle. Certain characteristics of a user’s system can be used to distinguish groups of users from each other; by collecting enough such information, an individual user’s browser’s “digital fingerprint” can be computed, which can be as good, if not better, as an IP address in ascertaining which requests are from the same user.
Grouping requests in this manner, especially across multiple sites, can be used for both benign (and even arguably positive) purposes, as well as for malevolent purposes. An example of a reasonably benign purpose would be determining whether a particular person seems to prefer sites with dog illustrations as opposed to sites with cat illustrations (based on how often they visit the sites in question) and then automatically using the preferred illustrations on subsequent visits to participating sites. Malevolent purposes, however, could include governments combining information such as the person’s home address (determined from the addresses they use when getting driving directions on one site) with their apparent political affiliations (determined by examining the forum sites that they participate in) to determine whether the person should be prevented from voting in an election.
Since the malevolent purposes can be remarkably evil, user agent implementors are encouraged to consider how to provide their users with tools to minimize leaking information that could be used to fingerprint a user.
Unfortunately, as the first paragraph in this section implies, sometimes there is great benefit to be derived from exposing the very information that can also be used for fingerprinting purposes, so it’s not as easy as simply blocking all possible leaks. For instance, the ability to log into a site to post under a specific identity requires that the user’s requests be identifiable as all being from the same user, more or less by definition. More subtly, though, information such as how wide text is, which is necessary for many effects that involve drawing text onto a canvas (e.g. any effect that involves drawing a border around the text) also leaks information that can be used to group a user’s requests. (In this case, by potentially exposing, via a brute force search, which fonts a user has installed, information which can vary considerably from user to user.)
Features in this specification which can be used to fingerprint the user are marked as this paragraph is.(This is a fingerprinting vector.)
Other features in the platform can be used for the same purpose, though, including, though not limited to:
  • The exact list of which features a user agents supports.
  • The maximum allowed stack depth for recursion in script.
  • Features that describe the user’s environment, like Media Queries and the Screen object. [MQ] [CSSOMVIEW]
  • The user’s time zone.

1.11 A quick introduction to HTML

Ready for first implementations
This section is non-normative.
A basic HTML document looks like this:

Sample page

Sample page

This is a simple sample.

HTML documents consist of a tree of elements and text. Each element is denoted in the source by a start tag, such as ““, and an end tag, such as ““. 
(Certain start tags and end tags can in certain cases be omitted and are implied by other tags.)
Tags have to be nested such that elements are all completely within each other, without overlapping:

This is very wrong!

This is correct.

This specification defines a set of elements that can be used in HTML, along with rules about the ways in which the elements can be nested.
Elements can have attributes, which control how the elements work. In the example below, there is a hyperlink, formed using the a element and itshref attribute:
Attributes are placed inside the start tag, and consist of a name and a value, separated by an “=” character. The attribute value can remainunquoted if it doesn’t contain space characters or any of " ' ` = < or >. Otherwise, it has to be quoted using either single or double quotes. The value, along with the “=” character, can be omitted altogether if the value is the empty string.

HTML user agents (e.g. Web browsers) then parse this markup, turning it into a DOM (Document Object Model) tree. A DOM tree is an in-memory representation of a document.
DOM trees contain several kinds of nodes, in particular a DocumentType node, Element nodes, Text nodes, Comment nodes, and in some cases ProcessingInstruction nodes.
The markup snippet at the top of this section would be turned into the following DOM tree:
  • DOCTYPE: html
  • html
    • head
      • #text⏎␣␣
      • title
        • #textSample page
      • #text⏎␣
    • #text⏎␣
    • body
      • #text⏎␣␣
      • h1
        • #textSample page
      • #text⏎␣␣
      • p
        • #textThis is a
        • a href=”demo.html
          • #textsimple
        • #textsample.
      • #text⏎␣␣
      • #commentthis is a comment
      • #text⏎␣⏎
The root element of this tree is the html element, which is the element always found at the root of HTML documents. It contains two elements,head and body, as well as a Text node between them.
There are many more Text nodes in the DOM tree than one would initially expect, because the source contains a number of spaces (represented here by “␣”) and line breaks (“⏎”) that all end up as Text nodes in the DOM. However, for historical reasons not all of the spaces and line breaks in the original markup appear in the DOM. In particular, all the whitespace before head start tag ends up being dropped silently, and all the whitespace after the body end tag ends up placed at the end of the body.
The head element contains a title element, which itself contains a Text node with the text “Sample page”. Similarly, the body element contains an h1 element, a p element, and a comment.

This DOM tree can be manipulated from scripts in the page. Scripts (typically in JavaScript) are small programs that can be embedded using thescript element or using event handler content attributes. For example, here is a form with a script that sets the value of the form’s outputelement to say “Hello World”:
<form name="main">
Result: <output name="result">
document.forms.main.elements.result.value = 'Hello World';

Each element in the DOM tree is represented by an object, and these objects have APIs so that they can be manipulated. For instance, a link (e.g. the a element in the tree above) can have its “href” attribute changed in several ways:
var a = document.links[0]; // obtain the first link in the document
a.href = 'sample.html'; // change the destination URL of the link
a.protocol = 'https'; // change just the scheme part of the URL
a.setAttribute('href', ''); // change the content attribute directly
Since DOM trees are used as the way to represent HTML documents when they are processed and presented by implementations (especially interactive implementations like Web browsers), this specification is mostly phrased in terms of DOM trees, instead of the markup described above.

HTML documents represent a media-independent description of interactive content. HTML documents might be rendered to a screen, or through a speech synthesizer, or on a braille display. To influence exactly how such rendering takes place, authors can use a styling language such as CSS.
In the following example, the page has been made yellow-on-blue using CSS.

Sample styled page

body { background: navy; color: yellow; }

Sample styled page

This page is just a demo.

For more details on how to use HTML, authors are encouraged to consult tutorials and guides. Some of the examples included in this specification might also be of use, but the novice author is cautioned that this specification, by necessity, defines the language with a level of detail that might be difficult to understand at first.

1.11.1 Writing secure applications with HTML

This section is non-normative.
When HTML is used to create interactive sites, care needs to be taken to avoid introducing vulnerabilities through which attackers can compromise the integrity of the site itself or of the site’s users.
A comprehensive study of this matter is beyond the scope of this document, and authors are strongly encouraged to study the matter in more detail. However, this section attempts to provide a quick introduction to some common pitfalls in HTML application development.
The security model of the Web is based on the concept of “origins”, and correspondingly many of the potential attacks on the Web involve cross-origin actions. [ORIGIN]
Not validating user input
Cross-site scripting (XSS)
SQL injection
When accepting untrusted input, e.g. user-generated content such as text comments, values in URL parameters, messages from third-party sites, etc, it is imperative that the data be validated before use, and properly escaped when displayed. Failing to do this can allow a hostile user to perform a variety of attacks, ranging from the potentially benign, such as providing bogus user information like a negative age, to the serious, such as running scripts every time a user looks at a page that includes the information, potentially propagating the attack in the process, to the catastrophic, such as deleting all data in the server.
When writing filters to validate user input, it is imperative that filters always be whitelist-based, allowing known-safe constructs and disallowing all other input. Blacklist-based filters that disallow known-bad inputs and allow everything else are not secure, as not everything that is bad is yet known (for example, because it might be invented in the future).
For example, suppose a page looked at its URL’s query string to determine what to display, and the site then redirected the user to that page to display a message, as in:
If the message was just displayed to the user without escaping, a hostile attacker could then craft a URL that contained a script element:
If the attacker then convinced a victim user to visit this page, a script of the attacker’s choosing would run on the page. Such a script could do any number of hostile actions, limited only by what the site offers: if the site is an e-commerce shop, for instance, such a script could cause the user to unknowingly make arbitrarily many unwanted purchases.
This is called a cross-site scripting attack.
There are many constructs that can be used to try to trick a site into executing code. Here are some that authors are encouraged to consider when writing whitelist filters:
  • When allowing harmless-seeming elements like img, it is important to whitelist any provided attributes as well. If one allowed all attributes then an attacker could, for instance, use the onload attribute to run arbitrary script.
  • When allowing URLs to be provided (e.g. for links), the scheme of each URL also needs to be explicitly whitelisted, as there are many schemes that can be abused. The most prominent example is “javascript:“, but user agents can implement (and indeed, have historically implemented) others.
  • Allowing a base element to be inserted means any script elements in the page with relative links can be hijacked, and similarly that any form submissions can get redirected to a hostile site.
Cross-site request forgery (CSRF)
If a site allows a user to make form submissions with user-specific side-effects, for example posting messages on a forum under the user’s name, making purchases, or applying for a passport, it is important to verify that the request was made by the user intentionally, rather than by another site tricking the user into making the request unknowingly.
This problem exists because HTML forms can be submitted to other origins.
Sites can prevent such attacks by populating forms with user-specific hidden tokens, or by checking Origin headers on all requests.
A page that provides users with an interface to perform actions that the user might not wish to perform needs to be designed so as to avoid the possibility that users can be tricked into activating the interface.
One way that a user could be so tricked is if a hostile site places the victim site in a small iframe and then convinces the user to click, for instance by having the user play a reaction game. Once the user is playing the game, the hostile site can quickly position the iframe under the mouse cursor just as the user is about to click, thus tricking the user into clicking the victim site’s interface.
To avoid this, sites that do not expect to be used in frames are encouraged to only enable their interface if they detect that they are not in a frame (e.g. by comparing the window object to the value of the top attribute).

1.11.2 Common pitfalls to avoid when using the scripting APIs

This section is non-normative.
Scripts in HTML have “run-to-completion” semantics, meaning that the browser will generally run the script uninterrupted before doing anything else, such as firing further events or continuing to parse the document.
On the other hand, parsing of HTML files happens asynchronously and incrementally, meaning that the parser can pause at any point to let scripts run. This is generally a good thing, but it does mean that authors need to be careful to avoid hooking event handlers after the events could have possibly fired.
There are two techniques for doing this reliably: use event handler content attributes, or create the element and add the event handlers in the same script. The latter is safe because, as mentioned earlier, scripts are run to completion before further events can fire.
One way this could manifest itself is with img elements and the load event. The event could fire as soon as the element has been parsed, especially if the image has already been cached (which is common).
Here, the author uses the onload handler on an img element to catch the load event:
If the element is being added by script, then so long as the event handlers are added in the same script, the event will still not be missed:

var img = new Image();
img.src = 'games.png';
img.alt = 'Games';
img.onload = gamesLogoHasLoaded;
// img.addEventListener('load', gamesLogoHasLoaded, false); // would work also
However, if the author first created the img element and then in a separate script added the event listeners, there’s a chance that theload event would be fired in between, leading it to be missed:

<!-- the 'load' event might fire here while the parser is taking a
break, in which case you will not see it! -->

var img = document.getElementById('games');
img.onload = gamesLogoHasLoaded; // might never fire!

1.12 Conformance requirements for authors

This section is non-normative.
Unlike previous versions of the HTML specification, this specification defines in some detail the required processing for invalid documents as well as valid documents.
However, even though the processing of invalid content is in most cases well-defined, conformance requirements for documents are still important: in practice, interoperability (the situation in which all implementations process particular content in a reliable and identical or equivalent way) is not the only goal of document conformance requirements. This section details some of the more common reasons for still distinguishing between a conforming document and one with errors.

1.12.1 Presentational markup

This section is non-normative.
The majority of presentational features from previous versions of HTML are no longer allowed. Presentational markup in general has been found to have a number of problems:
The use of presentational elements leads to poorer accessibility
While it is possible to use presentational markup in a way that provides users of assistive technologies (ATs) with an acceptable experience (e.g. using ARIA), doing so is significantly more difficult than doing so when using semantically-appropriate markup. Furthermore, even using such techniques doesn’t help make pages accessible for non-AT non-graphical users, such as users of text-mode browsers.
Using media-independent markup, on the other hand, provides an easy way for documents to be authored in such a way that they work for more users (e.g. text browsers).
Higher cost of maintenance
It is significantly easier to maintain a site written in such a way that the markup is style-independent. For example, changing the color of a site that uses  throughout requires changes across the entire site, whereas a similar change to a site based on CSS can be done by changing a single file.
Larger document sizes
Presentational markup tends to be much more redundant, and thus results in larger document sizes.
For those reasons, presentational markup has been removed from HTML in this version. This change should not come as a surprise; HTML4 deprecated presentational markup many years ago and provided a mode (HTML4 Transitional) to help authors move away from presentational markup; later, XHTML 1.1 went further and obsoleted those features altogether.
The only remaining presentational markup features in HTML are the style attribute and the style element. Use of the style attribute is somewhat discouraged in production environments, but it can be useful for rapid prototyping (where its rules can be directly moved into a separate style sheet later) and for providing specific styles in unusual cases where a separate style sheet would be inconvenient. Similarly, thestyle element can be useful in syndication or for page-specific styles, but in general an external style sheet is likely to be more convenient when the styles apply to multiple pages.
It is also worth noting that some elements that were previously presentational have been redefined in this specification to be media-independent:bihrssmall, and u.

1.12.2 Syntax errors

This section is non-normative.
The syntax of HTML is constrained to avoid a wide variety of problems.
Unintuitive error-handling behavior
Certain invalid syntax constructs, when parsed, result in DOM trees that are highly unintuitive.
For example, the following markup fragment results in a DOM with an hr element that is an earlier sibling of the correspondingtable element:

Errors with optional error recovery
To allow user agents to be used in controlled environments without having to implement the more bizarre and convoluted error handling rules, user agents are permitted to fail whenever encountering a parse error.
Errors where the error-handling behavior is not compatible with streaming user agents
Some error-handling behavior, such as the behavior for the 

... example mentioned above, are incompatible with streaming user agents (user agents that process HTML files in one pass, without storing state). To avoid interoperability problems with such user agents, any syntax resulting in such behavior is considered invalid.

Errors that can result in infoset coercion
When a user agent based on XML is connected to an HTML parser, it is possible that certain invariants that XML enforces, such as comments never containing two consecutive hyphens, will be violated by an HTML file. Handling this can require that the parser coerce the HTML DOM into an XML-compatible infoset. Most syntax constructs that require such handling are considered invalid.
Errors that result in disproportionally poor performance
Certain syntax constructs can result in disproportionally poor performance. To discourage the use of such constructs, they are typically made non-conforming.
For example, the following markup results in poor performance, since all the unclosed i elements have to be reconstructed in each paragraph, resulting in progressively more elements in each paragraph:

He dreamt.

He dreamt that he ate breakfast.

Then lunch.

And finally dinner.

The resulting DOM for this fragment would be:
  • p
    • i
      • #textHe dreamt.
  • p
    • i
      • i
        • #textHe dreamt that he ate breakfast.
  • p
    • i
      • i
        • i
          • #textThen lunch.
  • p
    • i
      • i
        • i
          • i
            • #textAnd finally dinner.
Errors involving fragile syntax constructs
There are syntax constructs that, for historical reasons, are relatively fragile. To help reduce the number of users who accidentally run into such problems, they are made non-conforming.
For example, the parsing of certain named character references in attributes happens even with the closing semicolon being omitted. It is safe to include an ampersand followed by letters that do not form a named character reference, but if the letters are changed to a string that does form a named character reference, they will be interpreted as that character instead.
In this fragment, the attribute’s value is “?bill&ted“:
Bill and Ted
In the following fragment, however, the attribute’s value is actually “?art©“, not the intended “?art&copy“, because even without the final semicolon, “&copy” is handled the same as “©” and thus gets interpreted as “©“:
Art and Copy
To avoid this problem, all named character references are required to end with a semicolon, and uses of named character references without a semicolon are flagged as errors.
Thus, the correct way to express the above cases is as follows:
Bill and Ted 
Art and Copy <!-- the & has to be escaped, since &copy is a named character reference -->
Errors involving known interoperability problems in legacy user agents
Certain syntax constructs are known to cause especially subtle or serious problems in legacy user agents, and are therefore marked as non-conforming to help authors avoid them.
For example, this is why the U+0060 GRAVE ACCENT character (`) is not allowed in unquoted attributes. In certain legacy user agents, it is sometimes treated as a quote character.
Another example of this is the DOCTYPE, which is required to trigger no-quirks mode, because the behavior of legacy user agents in quirks mode is often largely undocumented.
Errors that risk exposing authors to security attacks
Certain restrictions exist purely to avoid known security problems.
For example, the restriction on using UTF-7 exists purely to avoid authors falling prey to a known cross-site-scripting attack using UTF-7.
Cases where the author’s intent is unclear
Markup where the author’s intent is very unclear is often made non-conforming. Correcting these errors early makes later maintenance easier.
For example, it is unclear whether the author intended the following to be an h1 heading or an h2 heading:

Contact details

Cases that are likely to be typos
When a user makes a simple typo, it is helpful if the error can be caught early, as this can save the author a lot of debugging time. This specification therefore usually considers it an error to use element names, attribute names, and so forth, that do not match the names defined in this specification.
For example, if the author typed  instead of 

, this would be flagged as an error and the author could correct the typo immediately.
Errors that could interfere with new syntax in the future
In order to allow the language syntax to be extended in the future, certain otherwise harmless features are disallowed.
For example, “attributes” in end tags are ignored currently, but they are invalid, in case a future change to the language makes use of that syntax feature without conflicting with already-deployed (and valid!) content.
Some authors find it helpful to be in the practice of always quoting all attributes and always including all optional tags, preferring the consistency derived from such custom over the minor benefits of terseness afforded by making use of the flexibility of the HTML syntax. To aid such authors, conformance checkers can provide modes of operation wherein such conventions are enforced.

1.12.3 Restrictions on content models and on attribute values

This section is non-normative.
Beyond the syntax of the language, this specification also places restrictions on how elements and attributes can be specified. These restrictions are present for similar reasons:
Errors involving content with dubious semantics
To avoid misuse of elements with defined meanings, content models are defined that restrict how elements can be nested when such nestings would be of dubious value.
For example, this specification disallows nesting a section element inside a kbd element, since it is highly unlikely for an author to indicate that an entire section should be keyed in.
Errors that involve a conflict in expressed semantics
Similarly, to draw the author’s attention to mistakes in the use of elements, clear contradictions in the semantics expressed are also considered conformance errors.
In the fragments below, for example, the semantics are nonsensical: a separator cannot simultaneously be a cell, nor can a radio button be a progress bar.

Another example is the restrictions on the content models of the ul element, which only allows li element children. Lists by definition consist just of zero or more list items, so if a ul element contains something other than an li element, it’s not clear what was meant.
Cases where the default styles are likely to lead to confusion
Certain elements have default styles or behaviors that make certain combinations likely to lead to confusion. Where these have equivalent alternatives without this problem, the confusing combinations are disallowed.
For example, div elements are rendered as block boxes, and span elements as inline boxes. Putting a block box in an inline box is unnecessarily confusing; since either nesting just div elements, or nesting just span elements, or nesting span elements insidediv elements all serve the same purpose as nesting a div element in a span element, but only the latter involves a block box in an inline box, the latter combination is disallowed.
Another example would be the way interactive content cannot be nested. For example, a button element cannot contain atextarea element. This is because the default behavior of such nesting interactive elements would be highly confusing to users. Instead of nesting these elements, they can be placed side by side.
Errors that indicate a likely misunderstanding of the specification
Sometimes, something is disallowed because allowing it would likely cause author confusion.
For example, setting the disabled attribute to the value “false” is disallowed, because despite the appearance of meaning that the element is enabled, it in fact means that the element is disabled (what matters for implementations is the presence of the attribute, not its value).
Errors involving limits that have been imposed merely to simplify the language
Some conformance errors simplify the language that authors need to learn.
For example, the area element’s shape attribute, despite accepting both circ and circle values in practice as synonyms, disallows the use of the circ value, so as to simplify tutorials and other learning aids. There would be no benefit to allowing both, but it would cause extra confusion when teaching the language.
Errors that involve peculiarities of the parser
Certain elements are parsed in somewhat eccentric ways (typically for historical reasons), and their content model restrictions are intended to avoid exposing the author to these issues.
For example, a form element isn’t allowed inside phrasing content, because when parsed as HTML, a form element’s start tag will imply a p element’s end tag. Thus, the following markup results in two paragraphs, not one:

Welcome. Name:

It is parsed exactly like the following:


Errors that would likely result in scripts failing in hard-to-debug ways
Some errors are intended to help prevent script problems that would be hard to debug.
This is why, for instance, it is non-conforming to have two id attributes with the same value. Duplicate IDs lead to the wrong element being selected, with sometimes disastrous effects whose cause is hard to determine.
Errors that waste authoring time
Some constructs are disallowed because historically they have been the cause of a lot of wasted authoring time, and by encouraging authors to avoid making them, authors can save time in future efforts.
For example, a script element’s src attribute causes the element’s contents to be ignored. However, this isn’t obvious, especially if the element’s contents appear to be executable script — which can lead to authors spending a lot of time trying to debug the inline script without realizing that it is not executing. To reduce this problem, this specification makes it non-conforming to have executable script in a script element when the src attribute is present. This means that authors who are validating their documents are less likely to waste time with this kind of mistake.
Errors that involve areas that affect authors migrating to and from XHTML
Some authors like to write files that can be interpreted as both XML and HTML with similar results. Though this practice is discouraged in general due to the myriad of subtle complications involved (especially when involving scripting, styling, or any kind of automated serialization), this specification has a few restrictions intended to at least somewhat mitigate the difficulties. This makes it easier for authors to use this as a transitionary step when migrating between HTML and XHTML.
For example, there are somewhat complicated rules surrounding the lang and xml:lang attributes intended to keep the two synchronized.
Another example would be the restrictions on the values of xmlns attributes in the HTML serialization, which are intended to ensure that elements in conforming documents end up in the same namespaces whether processed as HTML or XML.
Errors that involve areas reserved for future expansion
As with the restrictions on the syntax intended to allow for new syntax in future revisions of the language, some restrictions on the content models of elements and values of attributes are intended to allow for future expansion of the HTML vocabulary.
For example, limiting the values of the target attribute that start with an U+005F LOW LINE character (_) to only specific predefined values allows new predefined values to be introduced at a future time without conflicting with author-defined values.
Errors that indicate a mis-use of other specifications
Certain restrictions are intended to support the restrictions made by other specifications.
For example, requiring that attributes that take media queries use only valid media queries reinforces the importance of following the conformance rules of that specification.

1.13 Suggested reading

This section is non-normative.
The following documents might be of interest to readers of this specification.
Character Model for the World Wide Web 1.0: Fundamentals [CHARMOD]
This Architectural Specification provides authors of specifications, software developers, and content developers with a common reference for interoperable text manipulation on the World Wide Web, building on the Universal Character Set, defined jointly by the Unicode Standard and ISO/IEC 10646. Topics addressed include use of the terms ‘character’, ‘encoding’ and ‘string’, a reference processing model, choice and identification of character encodings, character escaping, and string indexing.
Unicode Security Considerations [UTR36]
Because Unicode contains such a large number of characters and incorporates the varied writing systems of the world, incorrect usage can expose programs or systems to possible security attacks. This is especially important as more and more products are internationalized. This document describes some of the security considerations that programmers, system analysts, standards developers, and users should take into account, and provides specific recommendations to reduce the risk of problems.
Web Content Accessibility Guidelines (WCAG) 2.0 [WCAG]
Web Content Accessibility Guidelines (WCAG) 2.0 covers a wide range of recommendations for making Web content more accessible. Following these guidelines will make content accessible to a wider range of people with disabilities, including blindness and low vision, deafness and hearing loss, learning disabilities, cognitive limitations, limited movement, speech disabilities, photosensitivity and combinations of these. Following these guidelines will also often make your Web content more usable to users in general.
Authoring Tool Accessibility Guidelines (ATAG) 2.0 [ATAG]
This specification provides guidelines for designing Web content authoring tools that are more accessible for people with disabilities. An authoring tool that conforms to these guidelines will promote accessibility by providing an accessible user interface to authors with disabilities as well as by enabling, supporting, and promoting the production of accessible Web content by all authors.
User Agent Accessibility Guidelines (UAAG) 2.0 [UAAG]
This document provides guidelines for designing user agents that lower barriers to Web accessibility for people with disabilities. User agents include browsers and other types of software that retrieve and render Web content. A user agent that conforms to these guidelines will promote accessibility through its own user interface and through other internal facilities, including its ability to communicate with other technologies (especially assistive technologies). Furthermore, all users, not just users with disabilities, should find conforming user agents to be more usable.

4.2.3 The base element

The base element allows authors to specify the document base URL for the purposes of resolving relative URLs, and the name of the defaultbrowsing context for the purposes of following hyperlinks. The element does not represent any content beyond this information.Categories:
Metadata content.
Contexts in which this element can be used:
In a head element containing no other base elements.
Content model:
Tag omission in text/html:
No end tag.
Content attributes:
Global attributes
href — Document base URL
target — Default browsing context for hyperlink navigation and form submission
DOM interface:
interface HTMLBaseElement : HTMLElement {
attribute DOMString href;
attribute DOMString target;
There must be no more than one base element per document.
base element must have either an href attribute, a target attribute, or both.
The href content attribute, if specified, must contain a valid URL potentially surrounded by spaces.
base element, if it has an href attribute, must come before any other elements in the tree that have attributes defined as taking URLs, except the html element (its manifest attribute isn’t affected by base elements).
If there are multiple base elements with href attributes, all but the first are ignored.
The target attribute, if specified, must contain a valid browsing context name or keyword, which specifies which browsing context is to be used as the default when hyperlinks and forms in the Document cause navigation.
base element, if it has a target attribute, must come before any elements in the tree that represent hyperlinks.
If there are multiple base elements with target attributes, all but the first are ignored.
The href IDL attribute, on getting, must return the result of running the following algorithm:
  1. If the base element has no href content attribute, then return the document base URL and abort these steps.
  2. Let fallback base url be the Document‘s fallback base URL.
  3. Let url be the value of the href attribute of the base element.
  4. Resolve url relative to fallback base url (thus, the base href attribute isn’t affected by xml:base attributes or base elements).
  5. If the previous step was successful, return the resulting absolute URL and abort these steps.
  6. Otherwise, return the empty string.
The href IDL attribute, on setting, must set the href content attribute to the given new value.
The target IDL attribute must reflect the content attribute of the same name.
In this example, a base element is used to set the document base URL:

This is an example for the <base> element

Visit the archives.

The link in the above example would be a link to ““.

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