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Style sheet languages

Style sheet languages. Johan Jeuring Utrecht University. Presenting documents. Fixed document structure - Formatting info in document (HTML,MSWord) - Formatting info in style sheet (HTML+CSS)

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Style sheet languages

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  1. Style sheet languages Johan Jeuring Utrecht University

  2. Presenting documents • Fixed document structure- Formatting info in document (HTML,MSWord)- Formatting info in style sheet (HTML+CSS) • Generic document structure- Formatting info in style sheet (XML+CSS)- Function format :: Document_Structure –› Formats

  3. Formatting info in documentis bad • No reuse: to change the style of N docs, you have to edit all N docs. • The target type is fixed, so it is impossible to have a media dependent style. • Obfuscates the structure of a document.

  4. CSS: example <ARTICLE> <HEADLINE>Fredrick the Great meets Bach</HEADLINE> <AUTHOR>Johann Nikolaus Forkel</AUTHOR> <PARA> One evening, just as he was getting his <INSTRUMENT>flute</INSTRUMENT> ready and his musicians were assembled, an officer brought him a list of the strangers who had arrived. </PARA> </ARTICLE> INSTRUMENT { display: inline } ARTICLE, HEADLINE, AUTHOR, PARA { display: block } HEADLINE { font-size: 1.3em } AUTHOR { font-style: italic } ARTICLE, HEADLINE, AUTHOR, PARA { margin: 0.5em }

  5. Formatting info in style sheetis good • A single style sheet can be used for many docs. • Domain specific language for pretty printing. • Separation of concerns. • Structure remains clear.

  6. Function format :: Structure –› Formats Is usually written with the help of a pretty printing library: empty :: Formats text :: String –› Formats indent :: Int –› Formats –› Formats (>|<) :: Formats –› Formats –› Formats (>-<) :: Formats –› Formats –› Formats

  7. A function formathas its advantages and disadvantages • A lot of control and flexibility • Access to a programming language (Scheme, Haskell, OCaml, Lisp, ML, etc. have pretty printing libraries) • Complicated: you have to write and compile a program for pretty printing • Often limited set of `selectors’

  8. CSS: example <ARTICLE> <HEADLINE>Fredrick the Great meets Bach</HEADLINE> <AUTHOR>Johann Nikolaus Forkel</AUTHOR> <PARA> One evening, just as he was getting his <INSTRUMENT>flute</INSTRUMENT> ready and his musicians were assembled, an officer brought him a list of the strangers who had arrived. </PARA> </ARTICLE> INSTRUMENT { display: inline } ARTICLE, HEADLINE, AUTHOR, PARA { display: block } HEADLINE { font-size: 1.3em } AUTHOR { font-style: italic } ARTICLE, HEADLINE, AUTHOR, PARA { margin: 0.5em }

  9. CSS • A style sheet consists of rules: selector { declarations }. • A selector is an element name (h1) or an element name in a context (h1 p). • A declaration assigns a value to a property (font-size:32pt, color:blue, line-height:normal). • A large set of properties can be assigned.

  10. CSS2 • Many more selection options:- Wildcards: *- Child of a particular element: h1 > p- Following on an element: ul + p- First child: ul:first-child - Presence/value of attributes: (price[currency=“fl”])- ID selector: (Link#external) • Prefix and suffix generation: note:before {content: “NOTE: “; display:block}. • Media dependent style sheets: @media screen { em { ... } }, and print, handheld, aural, etc.

  11. CSS processing model • Create an abstract syntax tree from the source document. • Identify the target media type. • Retrieve all style sheets. • Compute the values of the properties of the elements by means of a `fold’ over the tree (using inheritance and cascading). The algebra has the following type:Rules –› Attributes –› Environment –› Tree+Attributes • Generate a formatting structure. • Show the formatting structure on the target medium.

  12. CSS: discussion 1 • Mostly simple, easy to use. • Limited control (no reordering, for example), limited expression power. For exampleEM { font-size=parent[parent].font-size * 1.2 }cannot be specified.

  13. CSS: discussion 2 <TITLE> A Document </TITLE> <P> PSL provides general language mechanisms: <OL> <LI> expressions, <LI> tree navigation functions, <LI> conditionals through an if-then-else construct, <LI> tree elaboration, <LI> access to both specified and actual layout, and <LI> constraint-based layout. </OL>

  14. CSS: discussion 2 <TITLE> A Document </TITLE> <P> PSL provides general language mechanisms: <OL> <LI id=a> expressions, <LI id=b> tree navigation functions, <LI id=c> conditionals through an if-then-else construct, <LI id=d> tree elaboration, <LI id=e> access to both specified and actual layout, and <LI id=f> constraint-based layout. </OL> LI { width : 200px; position: absolute; line-height:12px; } #a { top : 50; left : 0; } #b { top : 74px; left : 0; } #c { top : 98px; left : 0; } #d { top : 50; left : 200px; } #e { top : 74px; left : 200px; } #f { top : 98px; left : 200px; }

  15. CSS: discussion 3 • Complicated value language: P { font-size:12pt } P { font-size:medium } BLOCKQUOTE { font-size:larger } EM { font-size:200% } • Inconsistent use of value language:EM { line-height:200% }

  16. CSS: discussion 4 • No possibilities to specify constraints:- Suppose I want two boxes of minimal width, wide enough to fit two items, and one is twice as wide and high as the other.- Suppose I want a font-size at least 11pt, anything larger is fine (so a browser can decide which font to choose).

  17. Alternative style sheet languages • Constraint Cascading Style Sheets (CCSS) • PSL (Proteus Style Sheets) • XSL (XML Stylesheet Language)

  18. Constraint Cascading Style Sheets (CCSS)

  19. Styles as constraints <HTML> <HEAD> <TITLE> Table example </TITLE> <LINK REL=“stylesheet” HREF=“table.css” TYPE=“text/css”> </HEAD> <BODY> <TABLE ID=t> <COL ID=c1 CLASS=medcol> <COL ID=c2> <TR> <TD COLSPAN=2> <IMG ID=i1 SRC=“bach1.gif”></TD> </TR> <TR> <TD>Text1</TD> <TD>Text2</TD> </TR> </TABLE> </BODY> </HTML> #t[width] = #c1[width]+#c2[width] REQUIRED #c1[width] >= width(“Text1”) REQUIRED #c2[width] >= width(“Text2”) REQUIRED #c1[width]+#c2[width]>= #i1[width] REQUIRED #t[width] = 0 WEAK #c1[width] = 0.3*#t[width] DESIGNER .medcol { width:30% }

  20. Finite domain constraints <HTML> <HEAD> <TITLE> Simple example </TITLE> <LINK REL=“stylesheet” HREF=“simple.css” TYPE=“text/css”> </HEAD> <BODY> <H1 ID=h>Boring quotes</H1> <P ID=p>Stop reading this because <BLOCKQUOTE ID=q1>She said that <BLOCKQUOTE ID=q2>He said that </BLOCKQUOTE> </BLOCKQUOTE> </P> </BODY> </HTML> #h[font-size] Є {9,10,12,16,36,72} REQUIRED #p[font-size] Є {9,10,12,16,36,72} REQUIRED #q1[font-size] Є {9,10,12,16,36,72} REQUIRED #q2[font-size] Є {9,10,12,16,36,72} REQUIRED #h[font-size] = 13 DESIGNER #p[font-size] = 11 DESIGNER #q1[font-size] = 0.9 * #p[font-size] DESIGNER #q2[font-size] = 0.9 * #q1[font-size] DESIGNER H1 { font-size:13pt } P { font-size:11pt } BLOCKQUOTE { font-size:90% }

  21. Overlapping rules H1 { font-size:13pt } P { font-size:11pt } BLOCKQUOTE { font-size:90% } BLOCKQUOTE BLOCKQUOTE { font-size:100% } #h[font-size] = 13 <DESIGNER,0,0,1> #p[font-size] = 11 <DESIGNER,0,0,1> #q1[font-size] = 0.9 * #p[font-size] <DESIGNER,0,0,1> #q2[font-size] = 0.9 * #q1[font-size] <DESIGNER,0,0,1> #q2[font-size] = 1.0 * #q1[font-size] <DESIGNER,0,0,2>

  22. Solving constraints • There are 7 classes of constraints in CCSS: weak, browser, user, designer, designer-important, user-important, required. • The constraints consist of linear arithmetic equality and inequality constraints, which are solved by the Cassowary algorithm- incremental version of the simplex algorithm- minimizes the value of the `weighted-sum-better’ comparator. • The font constraints are solved by a separate dynamic programming algorithm

  23. The CCSS extension P { constraint: font-size <= (parent[parent])[font-size]+2 } @variable table-width; TABLE { constraint: width=table-width } @constraint #c1[width] = #c2[width] * {constraint: font-size>=12 !important }

  24. CCSS evaluation • Useful extension of CSS • Inherits most of the problems from CSS

  25. PSL

  26. PSL: An example Document: <HTML> <HEAD> <TITLE> A Sample HTML document </TITLE> <BODY> <H1> <A href=“http://www.w3.org/Style”>Style sheets</A> </H1> <UL> <LI> Give users control of formatting <LI> Support the accessibility goals of the <A href=“http://www.w3.org/WAI”>Web Accessibility Initiative</A> <LI> Support multiple presentations </UL> Desired presentation: Style sheets –› http://www.w3.org/Style Web Accessibility Initiative –› http://www.w3.org/WAI

  27. A PSL Style sheet ELABORATIONS { linebreak: Markup (“<BR>”) { visible=Yes; } arrow: Markup (“<IMG src=arrow-grey.gif>”) { visible=Yes; } url: Content (getAttribute(creator, “href”)) { visible=Yes; fontSize=12; } } DEFAULT { lineHeight = Self.fontSize*1.5; } RULES { HTML {visible=No; fgColor=“black”; fontSize=14 } A { if (getAttribute(self, “href”) != “”) then visible=Yes; gfColor=black; underlineNumber=1; createRight(arrow, url, linebreak); endif } }

  28. Properties and rules • Syntax: HTML {visible=No; fgColor=“black”; fontSize=14 } • General form for property rules:<property> = <expression>; • Expressions may contain references to attributes of other nodes:<node expression > . <property name>;where a node expression is a (composition of) Parent, LeftSib, RightSib, FirstChild, LastChild, NthChild, and a couple more.

  29. Order of evaluation • Node-specific rule • DEFAULT section rule • Inherited value: the rule <property> = Parent . <property>is added at each level for each property • Initial value

  30. Tree elaboration • In general, tree transformation is not possible • You can add nodes (Markup), text (Content), or pictures (Graphic) to a tree • These components can be added to the left (CreateLeft) or right (CreateRight) of the current node, or as the first (CreateFirst) or last (CreateLast) child:CreateRight(arrow, url, linebreak)

  31. Box Layout <position name> : <point name> = <expression> LI { if (ChildNum(Self) == round(NumChildren(Parent)/2+1) then VertPos: Top = Parent.Top; HorizPos: Left = LeftSib.Left+LeftSib.Width; else VertPos: Top = LeftSib.Actual Bottom; HorizPos: Left = LeftSib.Left; endif Width = 200; }

  32. PLS evaluation • Again, a useful extension of CSS • Inconsistent treatment of box layout • Limited expression power

  33. Extensible Stylesheet Language(XSL)

  34. What is XSL? • XSL - Styling using formatting objects. Often uses the FO DTD • XSLT - Transforming XML documents. You don’t want to author your documents with the formatting objects. XSLT is used to transform your source documents.- Add prefix or suffix text to the content- Remove, create, reorder and sort elements- Reuse elements elsewhere in the document- Transform to another format, or to HTML, or to an XML FO document

  35. XSL: an example <person> <firstname>Johan</firstname> <surname>Jeuring</surname> <address>Padualaan 14</address> <city>Utrecht</city> </person> Using CSS, this cannot be transformed into: Name: Jeuring Johan City: Utrecht <stylesheet> <template match=“person”> <apply-templates select=“surname”/> <apply-templates select=“firstname”/> <BR> <apply-templates select=“city”/> </template> </stylesheet> In XSL:

  36. XSL Stylesheets • An XSL stylesheet looks as follows:<stylesheet> ...</stylesheet>or with transform instead of style sheets • Each style sheet consists of a number of template rules<template match=“para”> ...</template>

  37. Recursive processing <para>An <emph>emphsized</emph> word.</para>. <template match=“para”> <apply-templates/> </template> <template match=“emph”> <apply-templates/> </template>

  38. Reordering children <template match=“person”> <p>Name : <apply-templates select=“surname”/> , <apply-templates select=“firstname”/> </BR> City: <apply-templates select=“city”/> </BR> </template> <template match=“intro”> <apply-templates select=“//chapter/title”/> </template>

  39. Creating new elements <template match=“person”> <apply-templates/> <element name=“contact”> ... </element> </template>

  40. Computing values <template match=“text()”> <value-of select=“.”/> </template> <variable name=“Colour”>red</variable> <h1>The colour is <value-of select=“$Colour”/>.</h1> <value-of select=“2001 - Birthdate[Year]”/>

  41. XSL expressions • Expressions are used for selecting nodes for processing (boolean expressions), selecting different processing ways, and generating text in the result tree. • Expressions can be built from boolean operators, number operators, string operations, and a predefined set of functions (sum, round, ceiling). It is an error to use other functions.

  42. Named templates <template name=“make_table”> <td> <font face=“Times, serif” color=“blue” size=“2”> <b> <value-of select=“.”/> </b> </font> </td> </template> <template match=“person”> <call-template name=“make_table”> </template>

  43. XSL Discussion • Relatively powerful, declarative style sheet language • Unattractive syntax • Inconsistent use of XML mark up (in rules, but not in expressions, for example) • No static checks, except for overlapping tags • The generated tree cannot be processed by the processing style sheet (`one flow over the tree’) • Limited form of definable first-order functions (named templates)

  44. Style sheet languages: conclusions • The development of style sheet languages for structured documents is an active research topic • CSS and XSL each have their serious limitations • CCSS adds constraints to CSS; a useful extension • PSL adds a simple, limited attribute grammar system to CSS, also a useful extension • I would like to have a combination of CCSS and PSL, combined with a more powerful expression language.

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