[TH tkhtml n]

[Section Name]
	tkhtml - Widget to render html documents.

[Section Synopsis]
	html pathName ?options?

[Section Standard Options]
	[Code {
		-height
		-width
		-xscrollcommand   
		-xscrollincrement
		-yscrollcommand   
		-yscrollincrement
	}]

	See the options(n) manual entry for details on the standard options.

[Section Widget-Specific Options]
	[Option defaultstyle {
		This option is used to set the default style-sheet for the
		widget. The option value should be the entire text of the
		default style-sheet.

		The default stylesheet defines things that are "built-in" to
		the document - for example the behaviour of <p> or <img> tags
		in html. The idea behind making it flexible is to allow
		Tkhtml to display anything that looks roughly like an XML
		document. But this will not work at the moment because of
		other assumptions the implementation makes about the set
		of valid tags. Currently, only valid HTML tags are recognized.

		The Tkhtml package adds the [SQ ::tkhtml::htmlstyle] command to
		the interpreter it is loaded into. Invoking this command
		returns a CSS document suitable for use with Tkhml as a default
		stylesheet for HTML documents. If the -quirks option is
		passed to [SQ ::tkhtml::htmlstyle] then the returned document
		includes some extra rules used when rendering legacy documents.

		If the value of the -defaultstyle option is changed, the new
		value does not take effect until after the next call to the
		widget [SQ reset] method.

		The default value of this option is the same as the string
		returned by the [SQ ::tkhtml::htmlstyle] command.
	}]
	[Option fontscale {
		This option is set to a floating point number, default 1.0.
		After CSS algorithms are used to determine a font size,
		it is multiplied by the value of this option. Setting this
		to a value other than 1.0 breaks standards compliance.
	}]
	[Option fonttable {
		This option must be set to a list of 7 integers. The first
		integer must be greater than 0 and each subsequent integer 
		must be greater than or equal to its predecessor.

		The seven integers define the sizes of the Tk fonts (in points)
		used when a CSS formatted document requests font-size
		'xx-small', 'x-small', 'small', 'medium', 'large', 'x-large' or
		'xx-large', respectively.

		The default value is {8 9 10 11 13 15 17}.
        }]
	[Option forcefontmetrics {
		This is a boolean option. If true, the font-metrics returned
		by Tk are overridden with calculated values based on the
		font requested. This improves CSS compatibility, but on some
		systems may cause problems. The default is true.
	}]
	[Option forcewidth {
		When determining the layout of a document, Tkhtml3 (and all
		other HTML/CSS engines) require as an input the width of
		the containing block for the whole document. For web-browsers,
		this is usually the width of the viewport in which the document
		will be displayed.

		If this option is true or the widget window is not mapped, 
		Tkhtml3 uses the value of the -width option as the initial
		containing block width. Otherwise, the width of the widget
		window is used.

		The default value is false.
	}]
	[Option imagecache {
		This boolean option (default true) determines whether or not
		Tkhtml3 caches the images returned to it by the -imagecmd
		callback script. If true, all images are cached until the
		next time the [SQ reset] sub-command is invoked. If false,
		images are discarded as soon as they are not in use.

		For simple applications, or applications that retrieve 
		images from local sources, false is usually a better value
		for this option (since it may save memory). However for
		web-browser applications where the background images of
		elements may be modified by mouseover events and so on, 
		true is a better choice.
	}]
	[Option imagecmd {
		As well as for replacing entire document nodes (i.e. <img>),
		images are used in several other contexts in CSS formatted
		documents, for example as list markers or backgrounds. If the
		 -imagecmd option is not set to an empty string (the default),
		then each time an image URI is encountered in the document, it
		is appended to the -imagecmd script and the resulting list
		evaluated.

		The command should return either an empty string, the name of a
		Tk image, or a list of exactly two elements, the name of a Tk
		image and a script. If the result is an empty string, then no
		image can be displayed. If the result is a Tk image name, then
		the image is displayed in the widget. When the image is no
		longer required, it is deleted. If the result of the command is
		a list containing a Tk image name and a script, then instead of
		deleting the image when it is no longer required, the script is
		evaluated.

		If the size or content of the image are modified while it is in
		use the widget display is updated automatically.
	}]
	[Option mode {
		This option may be set to "quirks", "standards" or 
		"almost standards", to set the rendering engine mode. The
		default value is "standards".

		TODO: List the differences between the three modes in Tkhtml.
	}]
	[Option parsemode {
		This option may be set to "html", "xhtml" or "xml", to set 
		the parser mode. The default value is "html".

		In "html" mode, the parser attempts to mimic the tag-soup
		approach inherited by modern web-browsers from the bad old
		days. Explicit XML style self-closing tags (i.e. closing
		a markup tag with "/>" instead of ">") are not handled
		specially. Unknown tags are ignored.

		"xhtml" mode is the same as "html" mode except that explicit
		self-closing tags are recognized.

		"xml" mode is the same as "xhtml" mode except that unknown
		tag names and XML CDATA sections are recognized.
	}]
	[Option shrink {
		This boolean option governs the way the widgets requested width
		and height are calculated. If it is set to false (the default),
		then the requested width and height are set by the -width
		and -height options as per usual.

		If this option is set to true, then the widgets requested width
		and height are determined by the current document. Each time
		the document layout is calculated, the widgets requested height
		and width are set to the size of the document layout. If the
		widget is unmapped when the layout is calculated, then the
		value of the -width option is used to determine the width
		of the initial containing block for the layout. Otherwise, the
		current window width is used.
	}]
	[Option zoom {
		This option may be set to any floating point number. Before
		the document layout is calculated, all lengths and sizes
		specified in the HTML document or CSS style configuration,
		implicit or explicit, are multiplied by this value.

		The default value is 1.0.
        }]
	[Option logcmd {
		This option is used for debugging the widget. It is not
		part of the official interface and may be modified or
		removed at any time. Don't worry about it.
	}]
	[Option timercmd {
		This option is used for debugging the widget. It is not
		part of the official interface and may be modified or
		removed at any time. Don't worry about it.
	}]
	[Option layoutcache {
		This option is used for debugging the widget. It is not
		part of the official interface and may be modified or
		removed at any time. Don't worry about it.

		If this boolean option is set to true, then Tkhtml caches 
		layout information to improve performance when the layout of a
		document must be recomputed. This can happen in a variety of
		situations, for example when extra text is appended to the
		document, a new style is applied to the document, a dynamic CSS
		selector (i.e. :hover) is activated, the widget window is
		resized, or when the size of an embedded image or Tk window
		changes.

		Layout caching consumes no extra memory or significant
		processing cycles, so in an ideal world there is no real reason
		to turn it off. But it can be a source of layout bugs, hence
		this option.

		The default value is true.
	}]

[Section Description]

	The [SQ html] command creates a new window (given by the pathName
	argument) and makes it into an html widget. The html command
	returns its pathName argument. At the time this command is invoked,
	there must not exist a window named pathName, but pathName's parent
	must exist.

[Section Widget Command]
	The [SQ html] command creates a new Tcl command whose name is
	pathName. This command may be used to invoke various operations on
	the widget as follows:

[Subcommand {
	pathName bbox _nodeHandle_
		If node nodeHandle generates content, this command returns a
		list of four integers that define the bounding-box of the
		generated content, relative to the top-left hand corner of the
		rendered document. The first two integers are the x and y
		coordinates of the top-left corner of the bounding-box, the
		later two are the x and y coordinates of the bottom-right
		corner of the same box. If the node does not generate content,
		then an empty string is returned.
}]

[Subcommand {
	pathName cget _option_
		Returns the current value of the configuration option given
		by option. Option may have any of the values accepted by
		the [SQ html] command.
}]

[Subcommand {
	pathName configure ?_option_? ?_value_?
		Query or modify the configuration options of the widget. If
		no option is specified, returns a list describing all of
		the available options for pathName (see Tk_ConfigureInfo
		for information on the format of this list). If option is
		specified with no value, then the command returns a list
		describing the one named option (this list will be
		identical to the corresponding sublist of the value
		returned if no option is specified). If one or more
		option-value pairs are specified, then the command modifies
		the given widget option(s) to have the given value(s); in
		this case the command returns an empty string. Option may
		have any of the values accepted by the [SQ html] command.
}]

[Subcommand {
	pathName fragment _html-text_
		TODO: Document this command.
}]

[Subcommand -4 {
	pathName handler node _tag_ _script_
	pathName handler attribute _tag_ _script_
	pathName handler script _tag_ _script_
	pathName handler parse _tag_ _script_
		This command is used to define "handler" scripts - Tcl
		callback scripts that are invoked by the widget when
		document elements of specified types are encountered. The
		widget supports two types of handler scripts: "node" and
		"script".

		For a "node" handler script, whenever a document element
		having the specified tag type (e.g. "p" or "link") is
		encountered during parsing, then the node handle for the
		node is appended to script and the resulting list
		evaluated as a Tcl command. See the section "NODE COMMAND"
		for details of how a node handle may be used to query and
		manipulate a document node. A node handler is called only
		after the subtree rooted at the node has been completely 
		parsed.

		If the handler script is a "script" handler, whenever a
		document node of type tag is parsed, two arguments are
		appended to the specified _script_ before it is evaluated.
		The first argument is a key-value list (suitable for passing 
		to the [SQ array set] command containing the HTML attributes
		that were part of the element declaration. The second
		argument is the literal text that appears between the 
		start and end tags of the element. 

		Elements for which a "script" handler is evaluated are not
		included in the parsed form of the HTML document. Instead,
		the result of the script handler evaluation is substituted
		into the document and parsed. For example, to handle the
		following embedded javascript:

[Code {
			<SCRIPT>
			  document.write("<p>A paragraph</p>")
			</SCRIPT>
}]

		a script handler that returns the string "<p>A paragraph</p>"
		must be configured for nodes of type "SCRIPT".

		Unlike node or script handlers, a "parse" handler may be
		associated with a specific opening tag, a closing tag or 
		with text tags (by specifying an empty string as the tag
		type). Whenever such a tag is encountered the parse handler
		script is invoked with two arguments, the node handle for
		the created node and the character offset of the in the parsed
		document. For a closing tag (i.e. "/form") an empty string
		is passed instead of a node handle.

		TODO: Describe "attribute" handlers.

		TODO: The offset values passed to parse handler scripts 
		currently have problems. See http://tkhtml.tcl.tk/cvstrac/tktview?tn=126

		Handler callbacks are always made from within 
		[SQ pathName parse] commands. The callback for a given node
		is made as soon as the node is completely parsed.  This can
		happen because an implicit or explicit closing tag is
		parsed, or because there is no more document data and the
		-final switch was passed to the [SQ pathName parse]
		command.

		TODO: Return values of handler scripts? If an exception
		occurs in a handler script?
}]

[Subcommand {
	pathName image
		This command returns the name of a new Tk image containing 
		the rendered document. Where Tk widgets would be mapped in a 
		live display, the image contains blank space.

		The returned image should be deleted when the script has 
		finished with it, for example:
[Code {
			set img \[.html image]
			# ... Use $img ...
			image delete $img
}]

		This command is included mainly for automated testing and 
		should be used with care, as large documents can result in very
		large images that take a long time to create and use vast
		amounts of memory.

		Currently this command is not available on windows. On that
		platform an empty string is always returned.
}]

[Subcommand {
	pathName node ? ?-index? _x_ _y_?
		This command is used to retrieve one or more document node
		handles from the current document. If the x and y parameters
		are omitted, then the handle returned is the root-node of the
		document, or an empty string if the document has no root-node
		(i.e. an empty document).

		If the x and y arguments are present, then a list of node 
		handles is returned. The list contains one handle for each
		node that generates content currently located at viewport
		coordinates (x, y). Usually this is only a single node, but
		floating boxes and other overlapped content can cause
		this command to return more than one node.  If no content is
		located at the specified coordinates or the widget window is
		not mapped, then an empty string is returned. 

		If the -index option is specified along with the x and y 
		coordinates, then instead of a list of node handles, a list of
		two elements is returned. The first element of the list is the
		node-handle associated with the generated text closest to 
		the specified (x, y) coordinates. The second list value is a
		byte (not character) offset into the text obtainable by 
		[SQ nodeHandle text] for the character closest to coordinates
		(x, y). The index may be used with the [SQ pathName tag] commands.

		The document node can be queried and manipulated using the
		interface described in the "NODE COMMAND" section.
}]

[Subcommand {
	pathName parse ?-final? _html-text_
		Append extra text to the end of the (possibly empty)
		document currently stored by the widget. 

		If the -final option is present, this indicates that the
		supplied text is the last of the document. Any subsequent
		call to [SQ pathName parse] before a call to 
		[SQ pathName reset] will raise an error. 

		If the -final option is not passed to [SQ pathName parse] along
		with the final part of the document text, node handler scripts
		for any elements closed implicitly by the end of the document
		will not be executed. It is not an error to specify an empty
		string for the _html-text_ argument.
}]

[Subcommand {
	pathName preload _uri_
		This command is only useful if the -imagecache option is
		set to true and an -imagecmd script is defined. It causes
		the widget to invoke the -imagecmd script to retrieve
		the image at URI _uri_. Assuming -imagecache is true, the
		returned image is then stored in the image-cache.

		This command may be useful when implementing scripting
		environments that support "preloading" of images.
}]

[Subcommand {
	pathName reset
		This is used to clear the internal contents of the widget
		prior to parsing a new document. The widget is reset such
		that the document tree is empty (as if no calls to 
		[SQ pathName parse] had ever been made) and no stylesheets
		except the default stylesheet are loaded (as if no
		invocations of [SQ pathName style] had occured).
}]

[Subcommand {
	pathName search _selector_
		The _selector_ argument passed to this command must be a valid
		CSS selector, for example "h1" or "a[SQ href]". This command
		returns a list of node-handles corresponding to the set of
		document nodes that match the supplied selector.
}]

[Subcommand {
	pathName style ?_options_? _stylesheet-text_
		Add a stylesheet to the widgets internal configuration. The
		stylesheet-text argument should contain the text of a
		complete stylesheet.  Incremental parsing of stylesheets is
		not supported, although of course multiple stylesheets may
		be added to a single widget.

		The following options are supported:
[Code {
			Option                   Default Value
			--------------------------------------
			-id <stylesheet-id>      "author"
			-importcmd <script>      ""
			-urlcmd    <script>      ""
}]

		The value of the -id option determines the priority taken
		by the style-sheet when assigning property values to
		document nodes (see chapter 6 of the CSS specification for
		more detail on this process).  The first part of the
		style-sheet id must be one of the strings "agent", "user"
		or "author". Following this, a style-sheet id may contain
		any text.  

		When comparing two style-ids to determine which stylesheet
		takes priority, the widget uses the following approach: If
		the initial strings of the two style-id values are not
		identical, then "user" takes precedence over "author", and
		"author" takes precedence over "agent". Otherwise, the
		lexographically largest style-id value takes precedence.
		For more detail on why this seemingly odd approach is
		taken, please refer to the "STYLESHEET LOADING" below.

		The -importcmd option is used to provide a handler script
		for @import directives encountered within the stylesheet
		text. Each time an @import directive is encountered, if the
		-importcmd option is set to other than an empty string, the
		URI to be imported is appended to the option value and the
		resulting list evaluated as a Tcl script. The return value
		of the script is ignored. If the script raises an error,
		then it is propagated up the call-chain to the 
		[SQ pathName style] caller.

		The -urlcmd option is used to supply a script to translate
		"url(...)" CSS attribute values. If this option is not set to
		"", each time a url() value is encountered the URI is appended
		to the value of -urlcmd and the resulting script evaluated. The
		return value is stored as the URL in the parsed stylesheet.
}]

[Subcommand -4 {
	pathName tag add _tag-name_ _node1_ _index1_ _node2_ _index2_
	pathName tag remove _tag-name_ _node1_ _index1_ _node2_ _index2_
	pathName tag configure _tag-name_ _option_ _value_ ?_option_ _value_...?
	pathName tag delete _tag-name_
		The [SQ pathName tag] command is used to highlight regions
		of text displayed by the widget. For example, a region
		of text selected using the pointer.

		Each displayed document character is identified by a 
		text node-handle (see below) and an index into the text
		returned by the [SQ node text] command. The index is a byte
		(not character) offset. See also the documentation for the 
		[SQ pathName node -index] command.  Both the [SQ pathName tag add]
		and [SQ pathName tag remove] use this convention.

		Evaluating the [SQ pathName tag add] command adds the specified
		tag to all displayed characters between the point in the 
		document described by (_node1_, _index1_) and the point
		described by (_node2_, _index2_). If the specified tag does
		not exist, it is created with default option values. The
		order in which the two specified points occur in the
		document is not important.

		The [SQ pathName tag remove] command removes the specified tag
		from all displayed characters between the point in the document
		described by (_node1_, _index1_) and the point described by
		(_node2_, _index2_). 

		The [SQ pathName tag configure] command is used to configure
		a tags options, which determine how tagged characters are
		displayed. If the specified tag does not exist, it is
		created. The following options are supported:
[Code {
			Option                   Default Value
			--------------------------------------
			-background              black
			-foreground              white
}]

		A tag can be completely deleted (removed from all characters
		and have it's option values set to the defaults) using the 
		[SQ pathName tag delete] command.

		The [SQ pathName tag] command replaces the 
		[SQ pathName select] command that was present in early
		alpha versions of Tkhtml3. Users should note that the
		options supported by [SQ pathName tag configure] are likely
		to change before beta release. See 
                http://tkhtml.tcl.tk/cvstrac/tktview?tn=73 (ticket #73).
}]

[Subcommand -4 {
	pathName text bbox _node1_ _index1_ _node2_ _index2_
	pathName text index _offset_ ?_offset_...?
	pathName text offset _node_ _index_
	pathName text text
		The [SQ pathName text] commands allow an application to
		query and interact with the text of the displayed document.
		This can be used, for example, to search for a string
		within an Html document, or to copy a region of text
		to the system clipboard.

		The [SQ pathName text text] command returns a string 
		containing the raw, unformatted text of the displayed document.
		Each block box is separated from the next by a newline
		character. Each block of whitespace is collapsed to a
		single space, except within blocks with the CSS 'white-space'
		property set to "pre".

		The [SQ pathName text index] command is used to transform
		from a character offset in the string returned by 
		[SQ pathName text text] to a node/index pair that can be
		used with the [SQ pathName tag] commands. The return value
		is a list of two elements, the node-handle followed by the
		index.

		Command [SQ pathName text offset] is the reverse of 
		[SQ pathName text index]. Given a node-handle and index of the
		type similar to that used by the [SQ pathName tag] commands,
		this command returns the corresponding character offset in the 
		string returned by [SQ pathName text text] command.
}]

[Subcommand -3 {
	pathName write continue
	pathName write text _html-text_
	pathName write wait
		TODO
}]

[Subcommand -3 {
	pathName xview
	pathName xview moveto _fraction_
	pathName xview scroll _number_ _what_
		This command is used to query or adjust the horizontal
		position of the viewport relative to the document layout.
		It is identical to the [SQ pathName xview] command
		implemented by the canvas and text widgets.
}]

[Subcommand -4 {
	pathName yview
	pathName yview moveto _fraction_
	pathName yview scroll _number_ _what_
	pathName yview _nodeHandle_
		This command is used to query or adjust the vertical
		position of the viewport relative to the document layout.
		It supports a superset of the [SQ pathName yview] interface
		implemented by the canvas and text widgets.

		As well as the standard interface copied from the canvas 
		and text widgets, Tkhtml supports passing a single node-handle
		as the only argument to [SQ pathName yview]. In this case 
		the viewport is scrolled so that the content generated by
		the node _nodeHandle_ is visible. This can be useful for
		implementing support for URI fragments.
}]

[Section Node Command]
	There are several interfaces by which a script can obtain a "node
	handle".  Each node handle is a Tcl command that may be used to
	access the document node that it represents. A node handle is valid
	from the time it is obtained until the next call to 
	[SQ pathName reset]. The node handle may be used to query and
	manipulate the document node via the following subcommands:

[Subcommand {
	nodeHandle attribute ??-default _default-value_? ?attribute? ?new-value??
		If the _attribute_ argument is present, then return the value
		of the named html attribute. If the attribute is not defined
		for the node, then a copy of the _default-value_ argument is
		returned instead. If no -default option was specified (and
		hence there is no _default-value_ argument) and the named
		attribute does not exist, an error is raised.

		If the _new-value_ argument is present, then set the named
                attribute to the specified _new-value_.

		If no _attribute_ argument is present, return a key-value list
		of the defined attributes of the form that can be passed to 
		[SQ array set].

	[Code {
		# Html code for node
		<p class="normal" id="second" style="color : red">

		# Value returned by [nodeHandle attr]
		{class normal id second style {color : red}}

		# Value returned by [nodeHandle attr class]
		normal
	}]
}]
[Subcommand {
	nodeHandle children
		Return a list of node handles for all children of nodeHandle.
		The leftmost child node becomes element 0 of the list, the
		second leftmost element 1, and so on.
}]
[Subcommand {
	nodeHandle destroy
		TODO. Experimental.
}]
[Subcommand -2 {
	nodeHandle dynamic set ?_flag_?
	nodeHandle dynamic clear ?_flag_?
		Set or clear a dynamic flag on a node. 

		The supported values for the _flag_ argument are "active",
		"hover", "focus", "link" and "visited". The status of each
		dynamic flag determines whether or not the corresponding CSS
		dynamic pseudo-classes are considered to match the node. For
		example, when the mouse moves over node $N, a script could
		invoke:
	[Code {
		$N dynamic set hover
	}]
		Or possibly, if $PN were the node the mouse hovered over
		previously:
	[Code {
		for {set n $PN} {$n ne ""} {set n [$n parent]} {
		        $n dynamic clear hover
		}
		for {set n $N} {$n ne ""} {set n [$n parent]} {
		        $n dynamic set hover
		}
	}]
}]
[Subcommand {
	nodeHandle insert ?-before _node_? _node-list_
		TODO. Experimental.
}]
[Subcommand {
	nodeHandle override ?_value_?
		TODO. Experimental.
}]
[Subcommand {
	nodeHandle parent
		Return the node handle for the node's parent. If the node
		does not have a parent (i.e. it is the document root), then
		return an empty string.
}]
[Subcommand {
	nodeHandle property ?-before|-after? ?property-name?
		TODO.
}]
[Subcommand {
	nodeHandle remove ?_node-list_?
		TODO.
}]
[Subcommand {
	nodeHandle replace ? ?options? newValue?
		This command is used to set and get the name of the
		replacement object for the node, if any. If the newValue
		argument is present, then this command sets the nodes
		replacement object name and returns the new value. If
		newValue is not present, then the current value is
		returned.

		A nodes replacement object may be set to the name of a Tk
		window or an empty string. If it is an empty string (the
		default and usual case), then the node is rendered normally.
		If the node replacement object is set to the name of a Tk
		window, then the Tk window is mapped into the widget in place
		of any other content (for example to implement form elements or
		plugins).

		The following options are supported:

[Code {
			Option                   Default Value
			--------------------------------------
			-deletecmd    <script>   ""
			-configurecmd <script>   ""
			-stylecmd     <script>   ""
}]

		When a replacement object is no longer being used by the
		widget (e.g. because the node has been deleted or 
		[SQ pathName reset] is invoked), the value of the
		-deletecmd option is evaluated as Tcl script. 

		If it is not set to an empty string (the default) each time
		the nodes CSS properties are recalculated, a serialized
		array is appended to the value of the -configurecmd option
		and the result evaluated as a Tcl command. The script
		should update the replacement objects appearance where
		appropriate to reflect the property values. The format of
		the appended argument is {p1 v1 p2 v2 ... pN vN} where the
		pX values are property names (i.e. "background-color") and
		the vX values are property values (i.e. "#CCCCCC"). The
		CSS properties that currently may be present in the array
		are listed below. More may be added in the future.

[Code {
			background-color    color
			font                selected
}]

		The value of the "font" property, if present in the
		serialized array is not set to the value of the
		corresponding CSS property. Instead it is set to the name
		of a Tk font determined by combining the various
		font-related CSS properties. Unless they are set to
		"transparent", the two color values are guaranteed to parse
		as Tk colors. The "selected" property is either true or
		false, depending on whether or not the replaced object is
		part of the selection or not. Whether or not an object is
		part of the selection is governed by previous calls to the
		[SQ pathName select] command.

		The -configurecmd callback is always executed at least once
		between the [SQ nodeHandle replace] command and when the
		replaced object is mapped into the widget display.
}]
[Subcommand {
	nodeHandle tag
		Return the name of the Html tag that generated this
		document node (i.e. "p" or "link"), or an empty string if
		the node is a text node.  
}]
[Subcommand {
	nodeHandle text ?-tokens|-pre?
		If the node is a "text" node, return the string contained
		by the node. If the node is not a "text" node, return an
		empty string.

		TODO: Document -tokens and -pre.
}]

[Section Stylesheet Loading]
	Apart from the default stylesheet that is always loaded (see the
	description of the -defaultstyle option above), a script may
	configure the widget with extra style information in the form of
	CSS stylesheet documents. Complete stylesheet documents (it is not
	possible to incrementally parse stylesheets as it is HTML document
	files) are passed to the widget using the [SQ pathName style]
	command.

	As well as any stylesheets specified by the application,
	stylesheets may be included in HTML documents by document authors
	in several ways: 

[Bulletlist {
		Embedded in the document itself, using a <style> tag. To
		handle this case an application script must register a
		"script" type handler for <style> tags using the 
		[pathName handler] command. The handler command should
		call [pathName style] to configure the widget with the
		stylesheet text.
} {
		Linked from the document, using a <link> tag. To handle
		this case the application script should register a "node"
		type handler for <link> tags.
} {
		Linked from another stylesheet, using the @import
		directive. To handle this, an application needs to
		configure the widget -importcommand option.
}]

[Code {
		# Implementations of application callbacks to load
		# stylesheets from the various sources enumerated above.
		# ".html" is the name of the applications tkhtml widget.
		# The variable $document contains an entire HTML document.
		# The pseudo-code <LOAD URI CONTENTS> is used to indicate
		# code to load and return the content located at $URI.

		proc script_handler {tagcontents} {
		    incr ::stylecount
		    set id "author.[format %.4d $::stylecount]"
		    set handler "import_handler $id"
		    .html style -id $id.9999 -importcmd $handler $tagcontents
		}

		proc link_handler {node} {
		    if {[node attr rel] == "stylesheet"} {
		        set URI [node attr href]
		        set stylesheet [<LOAD URI CONTENTS>]

		        incr ::stylecount
		        set id "author.[format %.4d $::stylecount]"
		        set handler "import_handler $id"
		        .html style -id $id.9999 -importcmd $handler $stylesheet
		    }
		}

		proc import_handler {parentid URI} {
		    set stylesheet [<LOAD URI CONTENTS>]

		    incr ::stylecount
		    set id "$parentid.[format %.4d $::stylecount]"
		    set handler "import_handler $id"
		    .html style -id $id.9999 -importcmd $handler $stylesheet
		}

		.html handler script style script_handler
		.html handler node link link_handler

		set ::stylecount 0

		.html parse -final $document
}]

	The complicated part of the example code above is the generation of
	stylesheet-ids, the values passed to the -id option of the 
	[SQ .html style] command. Stylesheet-ids are used to determine the
	precedence of each stylesheet passed to the widget, and the role it
	plays in the CSS cascade algorithm used to assign properties to
	document nodes. The first part of each stylesheet-id, which must be
	either "user", "author" or "agent", determines the role the
	stylesheet plays in the cascade algorithm. In general, author
	stylesheets take precedence over user stylesheets which take
	precedence over agent stylesheets. An author stylesheet is one
	supplied or linked by the author of the document. A user stylesheet
	is supplied by the user of the viewing application, possibly by
	configuring a preferences dialog or similar. An agent stylesheet is
	supplied by the viewing application, for example the default
	stylesheet configured using the -defaultstyle option.

	The stylesheet id mechanism is designed so that the cascade can be
	correctly implemented even when the various stylesheets are passed
	to the widget asynchronously and out of order (as may be the case
	if they are being downloaded from a network server or servers).

[Code {
		#
		# Contents of HTML document
		#

		<html><head>
		    <link rel="stylesheet" href="A.css">
		    <style>
		        @import uri("B.css")
		        @import uri("C.css")
		        ... rules ...
		    </style>
		    <link rel="stylesheet" href="D.css">
		... remainder of document ...

		#
		# Contents of B.css
		#
		
		@import "E.css"
		... rules ...
}]

	In the example above, the stylesheet documents A.css, B.css, C.css,
	D.css, E.css and the stylesheet embedded in the <style> tag are all
	author stylesheets. CSS states that the relative precedences of the
	stylesheets in this case is governed by the following rules:

[Bulletlist {
		Linked, embedded or imported stylesheets take precedence
		over stylesheets linked, embedded or imported earlier in
		the same document or stylesheet.
} {
		Rules specified in a stylesheet take precedence over rules
		specified in imported stylesheets.
}]

	Applying the above two rules to the example documents indicates
	that the order of the stylesheets from least to most important is:
	A.css, E.css, B.css, C.css, embedded <stylesheet>, D.css. For the
	widget to implement the cascade correctly, the stylesheet-ids
	passed to the six [SQ pathName style] commands must sort
	lexigraphically in the same order as the stylesheet precedence
	determined by the above two rules. The example code above shows one
	approach to this. Using the example code, stylesheets would
	be associated with stylesheet-ids as follows: 

[Code {
		Stylesheet         Stylesheet-id
		-------------------------------
		A.css              author.0001.9999
		<embedded style>   author.0002.9999
		B.css              author.0002.0003.9999
		E.css              author.0002.0003.0004.9999
		C.css              author.0002.0005.9999
		D.css              author.0006.9999
}]

	Entries are specified in the above table in the order in which the
	calls to [SQ html style] would be made. Of course, the example code
	fails if 10000 or more individual stylesheet documents are loaded.
	More inventive solutions that avoid this kind of limitation are
	possible.

	Other factors, namely rule specificity and the !IMPORTANT directive
	are involved in determining the precedence of individual stylesheet
	rules. These are completely encapsulated by the widget, so are not
	described here. For complete details of the CSS cascade algorithm,
	refer to the CSS and CSS 2 specifications (www.w3.org).

[Section Orphan Nodes]