WD-insert-951221
WD-insert-951221This is a W3C Working Draft for review by W3C members and other interested parties. It is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress". A list of current W3C working drafts can be found at: http://www.w3.org/pub/WWW/TR
Note: since working drafts are subject to frequent change, you are advised to reference the above URL, rather than the URLs for working drafts themselves.
The HyperText Markup Language (HTML) is a simple markup language used to create hypertext documents that are portable from one platform to another. HTML documents are SGML documents with generic semantics that are appropriate for representing information from a wide range of applications. This specification extends HTML to support the insertion of multimedia objects including Java applets, Microsoft Component Object Model (COM) objects (e.g. OLE Controls and OLE Document embeddings), and a wide range of other media plug-ins. The approach allows objects to be specified in a general manner and provides the ability to override the default implementation of objects.
HTML 2.0 defined only a single mechanism for inserting media into HTML documents: the IMG tag. While this tag has certainly proved worthwhile, the fact that it is restricted to image media severely limits it usefulness as richer and richer media finds its way onto the Web.
Developers have been experimenting with ideas for dealing with new media: Microsoft's DYNSRC attribute for video and audio, Netscape's EMBED tag for compound document embedding, and Sun's APP and APPLET tags for executable code.
Each of these proposed solutions attacks the problem from a slightly different perspective, and on the surface are each very different. In addition, each of these proposals falls short, in one way or another, of meeting the requirements of the Web community as a whole. However, we believe that this problem can be addressed with a single extension that addresses all of the current needs, and is fully extensible for the future.
This specification defines a new tag <INSERT> which subsumes the role of the IMG tag, and provides a general solution for dealing with new media, while providing for effective backwards compatibility with existing browsers. INSERT allows the HTML author to specify the data, including persistent data and/or properties/parameters for initializing objects to be inserted into HTML documents, as well as the code that can be used to display/manipulate that data. Here, the term object is used to describe the things that people want to place in HTML documents, but other terms for these things are: components, applets, plug-ins, media handlers, etc.
The data can be specified in one of several ways: via a universally unique object identifier (uuid) (<<REFERENCE OSF/DCE RPC Specification>>), a file specified by a URL, in-line data, or as a set of named properties. In addition, there are a number of attributes that allow authors to specify standard properties such as width, and height. The code for the object is specified in several ways: indirectly by the object's uuid, by information included as part of the object's data, and the combination of an object class name and a network address.
This specification covers the syntax and semantics for inserting such objects into HTML documents, but leaves out the architectural and application programming interface issues for how objects communicate with the document and other objects on the same page. It is anticipated that future specifications will cover these topics, including scripting languages and interfaces.
This section is intended to help readers get the feel of the insertion mechanism, and is not a normative part of the specification. The INSERT tag provides a richer alternative to the IMG tag. It may be used when the author wishes to provide an alternative for user agents that don't support a particular media. A simple example of using INSERT is:
<insert data=TheEarth.avi
type="application/avi">
<param name=loop value=infinite>
<img src=TheEarth.gif alt="The Earth">
</insert>
Here the user agent would show an animation if it supports the AVI format, otherwise it would show a GIF image. The IMG element is used for the latter as it provides for backwards compatibility with existing browsers. The TYPE attribute allows the user agent to quickly detect that it doesn't support a particular format, and hence avoid wasting time downloading the object. Another motivation for using the TYPE attribute is when the object is loaded off a CD-ROM, as it allows the format to be specified directly rather than being inferred from the file extension.
The next example inserts an OLE control for a clock:
<insert
id=clock1
type="application/x-oleobject"
data="http://www.foo.bar/test.stm"
code="http://www.foo.bar/controls"
>
</insert>
The ID attribute allows other controls on the same page to locate the clock. The DATA attribute points to the persistent stream data used to initialize the object's state. It includes a class identifier. The CODE attribute points to a file containing the implementation for this object. The file may contain the code for several classes, but this can be resolved by the class id from the object's data stream.
In the absence of the CODE attribute, the class identifier may be sufficient to locate the code implementing this object. User agents may provide a range of mechanisms for locating and downloading such code. For some formats such as image files, the Internet media type returned with the data is sufficient.
The class identifier can be specified explicitly using the CLASSID attribute. This value takes precedence over a class identifier included as part of the object's data, e.g.
<insert
id=clock1
type="application/x-oleobject"
classid="uuid:{663C8FEF-1EF9-11CF-A3DB-080036F12502}"
data="http://www.acme.com/ole/clock.stm"
>
</insert>
For speedy loading of objects you can inline the object's state data using the new URL scheme "data:", e.g.
<insert
id=clock1
classid="uuid:{663C8FEF-1EF9-11CF-A3DB-080036F12502}"
data="data:34hqi6n3gs9c3hdish2h568fhsb3uds7b4jawkl5h"
type="application/x-oleobject; clsid=no"
>
</insert>
The data is expressed as a Base64 encoded byte stream. The interpretation of this stream is class dependent. If the CLASSID attribute is missing or is insufficient to disambiguate the precise format of this stream then the TYPE attribute may be used to resolve matters. In the example, the Internet media type for COM streams takes a parameter that indicates that the stream doesn't start with a class identifier.
The next example is a Java applet:
<insert code="BounceItem.class" width=500 height=300>
</insert>
This is the bouncing heads demo. The implementation is specified by a relative URL and dereferenced with respect to the document URL. The images for the heads are loaded by the Java program as follows:
bounceimages[i-1] = getImage(getCodeBase(), "images/jon/T" + i + ".gif");
The getCodebase() function returns the URL used to retrieve the Java program, and is used here to dereference the relative URL (e.g. "images/jon/T1.gif") used for each image.
The semantics of the CLASSID attribute and the Java name space are still under discussion. The following is therefore liable to change. The previous example could have used the CLASSID attribute to specify the class name, e.g. CLASSID="java:BounceItem.class". The user agent would then use the Java specific mechanism to locate the code for this class, in this case, to first check if BounceItem.class is already installed, and if not to treat it as a relative URL, which is dereferenced with respect to the URL supplied by the CODE attribute if present, otherwise with respect to the document URL, e.g.
<insert
classid="java:NervousText.class"
code="http://java.acme.com/applets/NervousText.class"
width=400
height=75
align=baseline
>
<param name=text value="This is the Applet Viewer">
</insert>
In the absence of the CLASSID, user agents may obtain the Java class name from information supplied with the code itself or from the URL used to retrieve the code. The other attributes on the INSERT element define rendering properties of the container for the applet viewer. The PARAM element specifies a named property which is used to initialize the class. PARAM elements can be combined with data streams for greater control.
The document type definition provides the formal definition of the allowed syntax for HTML inserts. The following is an annotated listing of the DTD defining the semantics of the elements and their attributes. The complete listing appears at the end of this document.
Length values can be specified as an integer representing the number of screen pixels, or as a percentage of the current displayable region, for widths, this is the space between the current left and right margins, while for heights, this is the height of the current window or table cell etc.
Additional units may be added in the future to give designers greater control over the size of objects relative to fonts specified in fixed units such as points, picas, inches and centimeters.
Note: Shouldn't we add these now for consistency with HTML3 tables? The additional implementation effort would be minimal - with pt for points, pi for picas, in for inches, and cm for centimeters, where 72pt = 6pi = 1in = 2.54cm.
An opposing position is to use a style sheet language to specify the width and height, e.g. with the STYLE attribute or indirectly via the ID attribute. Perhaps the WIDTH and HEIGHT attributes should be considered as forming part of the style sheet? In which case, it would make sense to add the desired units (CSS uses the same notation as proposed above).
The INSERT element is used to insert an object into an HTML document. It requires both start and end tags. The INSERT element has the same content model as the HTML BODY element, except that one or more optional PARAM or ALIAS elements can be placed immediately after the INSERT start tag and used to initialize the inserted object. The content of the INSERT element is rendered if the object specified by the data, code or classid attributes can't be rendered. This provides for backwards compatibility with existing browsers, and allows authors to specify alternative media via nested INSERT elements.
Note that this doesn't provide the same level of flexibility as would be provided by a richer description of resource variants. For instance when a resource in available are several media types and for each such type in English, Spanish, French and German.
<!-- Content model entities imported from parent DTD:
%body.content allows inserts to contain headers, paras,
lists, form elements and even arbitrarily nested inserts.
-->
<!ENTITY % attrs
"id ID #IMPLIED -- element identifier --
class NAMES #IMPLIED -- for subclassing elements --
style CDATA #IMPLIED -- rendering annotation --
dir (ltr|rtl) #IMPLIED -- I18N text direction --
lang NAME #IMPLIED -- as per RFC 1766 --">
<!ENTITY % URL "CDATA" -- universal resource locator -->
<!ENTITY % Align
"(top|middle|textmiddle|bottom|left|center|right)">
<!ENTITY % Length "CDATA" -- standard length value -->
<!-- INSERT is a character-like element for inserting objects -->
<!ELEMENT insert - - (param*, bodytext)>
<!ATTLIST insert
%attrs -- id, class, style, lang, dir --
classid %URL #IMPLIED -- object's UUID --
code %URL #IMPLIED -- ref to object's code --
data %URL #IMPLIED -- ref to object's data --
type CDATA #IMPLIED -- Internet media type --
align %Align #IMPLIED -- positioning inside document --
height %Length #IMPLIED -- suggested height --
width %Length #IMPLIED -- suggested width --
border %Length #IMPLIED -- suggested link border width --
hspace %Length #IMPLIED -- suggested horizontal gutter --
vspace %Length #IMPLIED -- suggested vertical gutter --
usemap %URL #IMPLIED -- ref to image map --
ismap (ismap) #IMPLIED -- use server image map --
>
<!-- the BODYTEXT element is needed to avoid problems with
SGML mixed content, but is never used in actual documents -->
<!ELEMENT bodytext O O %body.content>
In general, all attribute names and values in this specification are case insensitive, except where noted otherwise. INSERT has the following attributes:
en, en-US, en-uk, i-cherokee, x-pig-latin.
The DIR attribute specifies an encapsulation boundary which governs the interpretation of neutral and weakly directional characters. It does not override the directionality of strongly directional characters. The DIR attribute value is one of LTR for left to right, or RTL for right to left, e.g. DIR=RTL.
The CLASSID attribute value takes the form of a URL scheme prefix separated by a colon from the character string defining the class identifier. The prefix is used to identify the object system for the class identifier, for example classid="uuid:{663C8FEF-1EF9-11CF-A3DB-080036F12502}" gives the uuid for a Microsoft COM object, using the UUID name space, while classid="java:Animator.class" gives the class name for Java applet.
CLASSID may be sufficient for the user agent to locate the code implementing the object. However, the CODE attribute can also be used with CLASSID to provide a hint as to where to look for this code. The search mechanism will in general depend on the object system the identifier belongs to. Note that the value specified with CLASSID takes precedence over a class identifier derived from the object's data stream.
When searching for the implementation of an object, the CLASSID attribute takes precedence over the CODE attribute which in turn takes precedence over the DATA attribute. A decision tree giving further details on this resolution procedure appears later on in this specification. In the absence of CLASSID a value for the class identifier may be derivable from the CODE or DATA attributes, for instance the Internet media type for the DATA may sufficient, e.g. when the data is for a GIF encoded image.
On its own, this would be meaningless. If the DATA attribute appears without a CODE or CLASSID attribute, then a TYPE attribute may be sufficient to interpret the data. For instance a Microsoft COM object can be asked to write its state using the WriteClassStream procedure. This inserts the object's class id as the first 16 bytes of the stream. If the TYPE attribute indicates that the data is in the COM persistent stream format, then the class id can be retrieved from the DATA attribute and used to find the code implementing the object's behaviour.
The CLASSID or CODE attributes can be used to override the default implementation as implied by the DATA attribute. For example, you may have the pickled data for an Excel spread sheet but want to view it with the "SuperGraph" package. You would then use the DATA attribute to point to the Excel spreadsheet data, and the CLASSID or CODE attribute to point to the SuperGraph plug-in.
The CLASSID, CODE and DATA attributes specify URLs. Any fragment identifier included as part of these URLs should be passed to the object, either directly, or by callback.
The following grammar for media types is a superset of that for MIME because it does not restrict itself to the official IANA and x-token types.
media-type = type "/" subtype *(";" parameter )
type = token
subtype = token
token = 1*<any (ASCII) CHAR except SPACE, CTLs, or tspecials>
tspecials = <one of the set> ( ) < > @ , ; : \ " / [ ] ? =
parameter = attribute "=" value
attribute = token
value = token | quoted-string
If a given media-type value has been registered by the IANA, any use of that value must be indicative of the registered data format. Although HTML allows the use of non-registered media types, such usage must not conflict with the IANA registry. Data providers are strongly encouraged to register their media types with IANA via the procedures outlined in RFC 1590.
All media-type's registered by IANA must be preferred over extension tokens. However, HTML does not limit applications to the use of officially registered media types, nor does it encourage the use of an "x-" prefix for unofficial types outside of explicitly short experimental use between consenting applications.
The following values are chosen for their ease of implementation, and their independence of other graphics occurring earlier on the same line:
For ALIGN=TEXTTOP, the top of the object is vertically aligned with the top of the current font.
For ALIGN=MIDDLE, the middle of the object is vertically aligned with the baseline.
For ALIGN=TEXTMIDDLE, the middle of the object is vertically aligned with the position midway between the baseline and the x-height for the current font. The x-height is defined as the top of a lower case x in western writing systems. If the text font is an all-caps style then use the height of a capital X. For other writing systems, align the middle of the object with the middle of the text.
For ALIGN=BASELINE, the bottom of the object is vertically aligned with the baseline of the text line in which the object appears.
For ALIGN=TEXTBOTTOM, the bottom of the object is vertically aligned with the bottom of the current font.
Note the proposed Netscape extensions for the align attribute of the IMG element are context sensitive, as are some of the implementions of ALIGN=TOP. See the test page at:
http://www.w3.org/pub/WWW/MarkUp/Group/imgtest.html
The following alignment values allow the object to float rather than being treated as part of the current line:
For ALIGN=LEFT, the object is floated down and over to the current left margin. Subsequent text is flowed past the right hand side of the visible area of the object.
For ALIGN=CENTER, the object is floated to after the end of the current line and centered between the left and right margins. Subsequent text starts at the beginning of the next line.
For ALIGN=RIGHT, the object is floated down and over to the current right margin. Subsequent text is flowed past the left hand side of the visible area of the object.
Smooth scaling a small image to a larger size provides an effective solution to reducing the time needed to download an image, offering better subjective results when compared to color reduction.
The PARAM element allows a list of named property values (used to initialize a OLE control, plug-in module or Java applet) to be represented as a sequence of PARAM elements. Note that PARAM is an empty element and should appear without an endtag.
<!ELEMENT param - O EMPTY -- named property value -->
<!ATTLIST param
name CDATA #REQUIRED -- property name --
value CDATA #IMPLIED -- property value --
valueref %URL #IMPLIED -- ref to object ALIAS --
type CDATA #IMPLIED -- Internet media type --
>
The NAME attribute defines the property name. The case sensitivity of the name is dependent on the code implementing the object.
The VALUE attribute is used to specify the property value. It is an opaque character string whose meaning is determined by the object based on the property name. Note that CDATA attribute values need characters such as & to be escaped using the standard SGML character entities, e.g. & for "&". It is also essential to escape the > character to defend against incorrect handling by many existing browsers (use >).
The VALUEREF attribute is used when the property is itself an object. A distinct attribute is needed as in some cases the property type cannot be deduced from the property name. VALUEREF typically provides a URL based reference to an ALIAS element (see below) that defines the object itself.
VALUEREF can also be used to specify an object directly. For example valueref=foo.gif. Another possibility is to use inline data with the "data:" URL scheme. Both of these options save having to include an associated ALIAS element.
The TYPE attribute is required when the "data:" URL scheme is used. It specifies the media type for the data stream and allows the user agent to decode the stream, e.g. to pick out an embedded class identifier.
The ALIAS element is used to define an object without inserting it into the document. It is used with the valueref attribute of the PARAM element to allow an object to be passed as parameter, when initializing an object associated with another INSERT or ALIAS element. The attributes take exactly the same meaning as for the INSERT element. The ALIAS element is a container and requires both start and end tags. It can be placed anywhere in the document HEAD or BODY. The contents are limited to PARAM and ALIAS elements, although it is anticipated that this may be extended to cover the same content model as INSERT at some point in the future.
<!-- ALIAS is allowed anywhere in document HEAD and BODY
it defines an alias for an object without inserting it -->
<!ELEMENT alias - - (param*, alias?)>
<!ATTLIST alias
id ID #REQUIRED -- defines name for alias --
data %URL #IMPLIED -- ref to object's data --
code %URL #IMPLIED -- ref to object's code --
classid %URL #IMPLIED -- object's UUID --
type CDATA #IMPLIED -- Internet media type --
>
Note that the object isn't created until its needed by something that references it (i.e. late binding). Each such reference when bound creates a separate copy of the object. In other words, the alias element is treated as a declaration for making an object. When another object is being created and has an alias as a parameter, the declaration defined by the alias is then used to create a new object. Notice that a given object class may be used by different aliases to create multiple instances of that class with different data.
If the aliased object isn't supported, or fails to load, the user agent should try the contents of the ALIAS element, which is currently restricted to another ALIAS element. The TYPE attribute can be used to specify the Internet Media Type for the object as a hint for this situation.
Like the INSERT, ALIAS elements are limited in scope to the document in which they appear. The objects created for these elements have a life time at least equal to that of the document. So if the user clicks on a link to move to another document, the objects are not destroyed and can be seen again when the user backtracks to the original document.
This section describes proposals for extending the capabilities of the insertion mechanism as an encouragement and guide to developers wishing to experiment with such features. These ideas are not part of the current specification, and support is not required for conformance with this specification.
Using a GIF image to tile the document background often results in significant delays while the image tile is downloaded. The ability to use a small Java applet or OLE Control to generate the image tile would allow rich background textures and patterns to be used without causing significant delay. As processing speeds increase, using an object to generate the background would make it practical to create animated backgrounds.
The proposed extension is to allow the BACKGROUND attribute of the BODY element to reference ALIAS elements, for example:
<title>Demo Document</title> <alias id=marble code="http://www.acme.com/applets/marble.class"> </alias> <body background="#marble"> <p>This document has a marble texture generated by an applet.
Overlays are useful for reducing network bandwidth needs. For instance, you can place a PNG overlay on top of a JPEG image. If the PNG image is used for an antialiased text overlay while the JPEG image is used for an underlying photographic image with a high compression factor, then the two images will take significantly less time to send than a single image combining both layers. Selecting the format and compression for each layer separately allows you to get higher compression for the same level of quality.
Overlays also save time by making caching more effective. For instance you might send a large image on one page, and then make small changes to it on subsequent pages. Using an overlay allows the original large image to be reused, so that only the small changes need to be sent with each successive page.
<!ELEMENT overlay - O EMPTY -- image overlay -->
<!ATTLIST overlay
id ID #IMPLIED -- for naming this overlay --
class NAMES #IMPLIED -- for subclassing element --
style CDATA #IMPLIED -- for attaching style info --
x %Length #IMPLIED -- offset from left of parent --
y %Length #IMPLIED -- offset from top of parent --
width %Length #IMPLIED -- suggested width --
height %Length #IMPLIED -- suggested height --
src %URL #IMPLIED -- network address of object --
>
For instance, here is a road map overlayed on an aerial photograph:
<insert data="photo.jpeg">
<overlay src=grid.png>
</insert>
The SRC attribute of the OVERLAY element could be used together with an ALIAS element. This allows you to create overlays from OLE controls or Java applets.
Many objects will size themselves according to their contents. Another popular feature is likely to be the ability for users to dynamically resize objects, e.g. by dragging size bars. The height and width attributes of the INSERT element can be used as suggested initial values. For instance, images can be automatically resized to match these values. The ability to smoothly magnify an image allows a small image file to fill a large space, and saves network time.
Simple user agents may consider objects as having a rectangular outline. Of course, the object can render itself with a transparent background to give the effect of a shaped object, but any text flowing past would still follow the rectangular frame around the object. Smarter user agents can "ask" the object for its outline and flow text around that.
To speed up display of the rest of the document, it is desirable to get the outline as early as possible. For this reason it makes sense to be able to specify the outine via an attribute (e.g. SHAPE) on the INSERT element. It is suggested that the user agent scale the object to the WIDTH and HEIGHT attribute values (if given) and then to clip it to the outline as given by the SHAPE attribute. Subsequent text would then flow around the outline. The ability to offset the object relative to the outline would also be very useful. The syntax for SHAPE is left to a future specification.
A time proven idiom for document layout is the figure. This is often an illustration, but may contain textual material separate from the main flow of the document. Figures are typically captioned and floated to between columns or to the top or bottom of a page. It is not uncommon to see separate contents lists for figures and tables, in addition to the main table of contents.
It is proposed that the FIG element is used to create captioned figures:
<!ENTITY % f.align "(left|center|right)">
<!ELEMENT fig - - (caption?, bodytext)>
<!ATTLIST fig
%attrs -- id, class, style, lang, dir --
align %f.align #IMPLIED -- position on page --
height %Length #IMPLIED -- suggested height --
width %Length #IMPLIED -- suggested width --
>
<!ENTITY % c.align "(top|bottom|left|right)">
<!ELEMENT caption - - %body.content>
<!ATTLIST caption
%attrs -- id, class, style, lang, dir --
align %c.align #IMPLIED -- position relative to figure --
>
For example:
<fig>
<caption>Mount Washington</caption>
<insert data=http://www.acme.com/images/vista.jpeg>
<p>A spectacular view of Mount Washington during a winter sunset.
</insert>
</fig>
Note that FIG is a block-like element similar to tables. If a user agent supports tables then adding support for figures is quite simple, since the FIG element behaves in the same as a table with a caption and a single cell. Of course some would argue that in this case, why not use the TABLE element. This is an example of approaching HTML with a view to getting a desired visual effect without regard to what the markup means. This makes it hard to export HTML to other document formats, and makes it harder to read the markup, as you now have to guess what the author actually was trying to do.
Image such as GIF and PNG formats allow textual messages to be included with the image data. This provides an appropriate place to store copyright messages since these will then be automatically transferred with the image when it is dragged from the document to the desktop etc. For other formats, copyright notices can be included in HTTP headers, or by wrapping up the objects as MIME multipart files.
Even when information can be included as part of the object's data, it is still useful to present a credit or copyright message to the user as part of the document text. Something immediately visible is more effective that something hidden! The suggested way of handling credits is to use a new character emphasis element CREDIT that can be given as part of the FIG contents, e.g.
<fig>
<insert data=http://www.acme.com/images/vista.jpeg>
<p>A spectacular view of Mount Washington during a winter
sunset.
</insert>
<credit>John Smith</credit>
</fig>
The DTD or document type definition provides the formal definition of the allowed syntax for HTML inserts.
<!-- Content model entities imported from parent DTD:
%body.content allows inserts to contain headers, paras,
lists, form elements and even arbitrarily nested inserts.
-->
<!ENTITY % attrs
"id ID #IMPLIED -- element identifier --
class NAMES #IMPLIED -- for subclassing elements --
style CDATA #IMPLIED -- rendering annotation --
dir (ltr|rtl) #IMPLIED -- I18N text direction --
lang NAME #IMPLIED -- as per RFC 1766 --">
<!ENTITY % URL "CDATA" -- universal resource locator -->
<!ENTITY % Align
"(top|middle|bottom|left|center|right)">
<!ENTITY % Length "CDATA" -- standard length value -->
<!-- INSERT is a character-like element for inserting objects -->
<!ELEMENT insert - - (param*, bodytext)>
<!ATTLIST insert
%attrs -- id, class, style, lang, dir --
data %URL #IMPLIED -- ref to object's data --
code %URL #IMPLIED -- ref to object's code --
classid %URL #IMPLIED -- object's UUID --
type CDATA #IMPLIED -- Internet media type --
align %Align #IMPLIED -- positioning inside document --
height %Length #IMPLIED -- suggested height --
width %Length #IMPLIED -- suggested width --
border %Length #IMPLIED -- suggested link border width --
hspace %Length #IMPLIED -- suggested horizontal gutter --
vspace %Length #IMPLIED -- suggested vertical gutter --
usemap %URL #IMPLIED -- ref to image map --
ismap (ismap) #IMPLIED -- use server image map --
>
<!-- the BODYTEXT element is needed to avoid problems with
SGML mixed content, but is never used in actual documents -->
<!ELEMENT bodytext O O %body.content>
<!ELEMENT param - O EMPTY -- named property value -->
<!ATTLIST param
name CDATA #REQUIRED -- property name --
value CDATA #IMPLIED -- property value --
valueref %URL #IMPLIED -- ref to object ALIAS --
type CDATA #IMPLIED -- Internet media type --
>
<!-- ALIAS is allowed anywhere in document HEAD and BODY
it defines an alias for an object without inserting it -->
<!ELEMENT alias - - (param*, alias?)>
<!ATTLIST alias
id ID #REQUIRED -- defines name for alias --
data %URL #IMPLIED -- ref to object's data --
code %URL #IMPLIED -- ref to object's code --
classid %URL #IMPLIED -- object's UUID --
type CDATA #IMPLIED -- Internet media type --
>
This section defines the steps needed to bind an object given various combinations of attributes. Without a precise semantics for this, different user agents would otherwise produce different results to the frustration of authors. The basic idea is that when finding the implementation for an object the CLASSID takes precedence over CODE, which in turn takes precedence over DATA.
In the following, the ExtractClassId function is somewhat simplified. This function depends on recognising the format of the data stream. Normally this will be given by the Internet Media Type for the stream. If the data is inlined, the format may be given by the TYPE attribute or derived from the CLASSID or CODE attributes. The format of the data stream also effects the Invoke function. For instance, the class initialization procedure may require you to adjust the stream pointer to after the class identifier.
This decision tree doesn't make explicit how to deal with the cases where you use the CODE attribute to load the implementation, but need additional information to find the appropriate entry point into this code. This could be provided by the CLASSID attribute or by a class identifier extracted from the data as pointed to by the DATA attribute.
properties = GetParams(); // collect properties from PARAM elements
type = AttributeValue(TYPE); // get value of TYPE attribute
if (clsid = AttributeValue(CLASSID)) then
if (class = LoadImplementation(clsid, null)) then
if (data = AttributeValue(DATA)) then
if (stream = LoadData(data)) then
Invoke(class, stream, params);
else
Fail("Can't load data");
fi
else // no data
Invoke(class, null, params);
fi
else // can't get implementation using only CLASSID
if (code = AttributeValue(CODE)) then
if (class = LoadImplementation(clsid, code)) then
if (data = AttributeValue(DATA)) then
if (stream = LoadData(data)) then
Invoke(class, stream, params);
else
Fail("Can't load data");
fi
else
Invoke(class, null, params);
fi
else // can't get implementation using classid/code
if (data = AttributeValue(DATA)) then
if (stream = LoadData(data)) then
if (clsid = ExtractClassId(stream)) then
if (class = LoadImplementation(clsid, code)) then
Invoke(class, stream, params);
else
Fail("Can't get implementation");
fi
else // no class id within data
Fail("Can't get implementation");
fi
else // can't load data
Fail("Can't get implementation");
fi
else // no DATA
Fail("Can't get implementation");
fi
fi
else // CLASSID not good enough and no CODE attribute
if (data = AttributeValue(DATA)) then
if (stream = LoadData(data)) then
if (clsid = ExtractClassId(stream)) then
if (class = LoadImplementation(clsid, null)) then
Invoke(class, stream, params);
else
Fail("Can't get implementation");
fi
else // no class id within data
Fail("Can't get implementation");
fi
else // can't load data
Fail("Can't load data");
fi
else // no DATA
Fail("Can't get implementation");
fi
fi
fi
else // no CLASSID
if (code = AttributeValue(CODE)) then
if (class = LoadImplementation(null, code)) then
if (data = AttributeValue(DATA)) then
if (stream = LoadData(data)) then
Invoke(class, stream, params);
else
Fail("Can't load data");
fi
else // no DATA
Invoke(class, null, params);
fi
else // CODE is insufficient to locate implementation
if (data = AttributeValue(DATA)) then
if (stream = Load(data)) then
if (clsid = ExtractClassId(stream)) then
if (class = LoadImplementation(clsid, code)) then
Invoke(class, stream, params);
else
Fail("Can't get implementation");
fi
else // no class id within data
Fail("Can't get implementation");
fi
else // can't load data
Fail("Can't get implementation");
fi
else // no DATA
Fail("Can't get implementation");
fi
fi
else // no CLASSID or CODE
if (data = AttributeValue(DATA)) then
if (stream = Load(data)) then
if (clsid = ExtractClassId(stream)) then
if (class = LoadImplementation(clsid, null)) then
Invoke(class, stream, params);
else
Fail("Can't get implementation");
fi
else // no class id within data
Fail("Can't get implementation");
fi
else // can't load data
Fail("Can't get implementation");
fi
else // no DATA
Fail("Can't get implementation");
fi
fi
fi
The markup language known as "HTML/2.0" provides for image maps. Image maps are document elements which allow clicking on different areas of an image to reference different network resources, as specified by Uniform Resource Locators (URIs). The image map capability in HTML/2.0 is limited in several ways, such as the restriction that it only works with documents served via the "HTTP" protocol, and the lack of a viable fallback for users of text-only browsers. This document specifies an extension to the HTML language, referred to as "Client-Side Image Maps," which resolves these limitations.
The World Wide Web Consortium: http://www.w3.org/pub/WWW/Consortium/