Lecture 7 the appearance of text
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Lecture 7 The Appearance of Text. CS 502: Computing Methods for Digital Libraries. Administration. Wednesday evening -- balance section sizes Assignment 2. The Design Tension. Who controls the appearance of a document stored in a repository and displayed on a client?

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

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CS 502: Computing Methods for Digital Libraries

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Lecture 7

The Appearance of Text

CS 502: Computing Methods for Digital Libraries


  • Wednesday evening -- balance section sizes

  • Assignment 2

The Design Tension

  • Who controls the appearance of a document stored in a repository and displayed on a client?

  • The representation in the repository -- page description, style sheets, fonts, etc.

  • The user -- window sizes, fonts and sizes, etc.

  • The rendering software (e.g., browser) -- mapping from markup to appearance.

The Design Tension

  • Important special cases:

  • The user has special requirements (e.g., poor eyesight).

  • The client computer has restricted capability (e.g., PalmPilot, slow network connection).

  • Presentation profile:

  • The representation in the repository suggests formatting, but client can over-ride the suggestions.

    • e.g., in HTML, alt option with img tag allows text string as substitute for image.

Style Sheet Systems

Markup languageStyle sheet systemNotes

SGMLDSSSLToo ambitious

for general use


HTMLCSSIn production

Cascading Style Sheets (CSS)




<title>Observations from the Library of Congress</title>


[See next slide]




[Body of document]



Cascading Style Sheets (CSS)



H1 { font-size: x-large; font-family: Arial, Helvetica, sans-serif;

color: #990000 }

H2 { font-size: large; font-family: Arial, Helvetica, sans-serif;

color: #990000}

H3 { font-family: Arial, Helvetica, sans-serif }

H4 { font-family: Arial, Helvetica, sans-serif; margin-left: 10px}

body { background: #fff7e7 }

P.nav {font-size: small; text-align: right }



Simple CSS

  • A rule defines styles to be applied to selected elements of a document, e.g.,

  • h1 {color: blue}



Simple CSS

  • A declaration has two parts, a property and a value, separated by a colon:

  • {color: blue}



  • More examples:

  • h1, h2 {font-family: sans-serif; color: blue}

  • h3 {font-family: Arial, Helvetica, sans-serif}

Simple CSS: Inheritance

  • HTML mark-up defines a hierarchy:

  • heading, paragraph, and list are elements of body

  • list items are elements within list

  • lists can be nested within each other.

  • CSS style sheets: If no rule explicitly selects an element, it inherits the rules for the elements higher in the hierarchy, e.g.

  • body {font-family: serif}

  • h1, h2 {font-family: sans-serif}

  • No explicit rule for paragraph or list -- inherit the styles that apply to body.

Simple CSS: Attaching Style Sheets


Embed style sheet in head of document

Provide link from document to external file

Browser or other rendering program

Built-in style sheet (usually implicit)


Private style sheet

Simple CSS: Cascading

  • Several style sheets may apply to the same page.

  • If the rules conflict:

  • one rule is selected; others are ignored

  • rules that explicitly select elements have priority over

  • inherited rules

  • rules can be flagged !important to gain priority

  • for direct conflict, the designer's rules have priority over

  • the user's

Use of CSS

  • Cascading style sheets have been available for several years.

  • They are supported by the major browsers.

  • Some major web sites use them (e.g., CNN).

  • Why are they not used more?

Page Description Languages

  • Objective:

  • To represent the appearance of documents with the same graphic quality as the best printed documents.

  • printed output

  • rendering on computer display

  • Secondary considerations:

  • storage and transmission

  • searching and manipulation


  • Macro language for high-quality printing; emphasis on mathematics

  • LaTex macro package

  • Metafont system for designing fonts

  • Specification and code made openly available by Donald Knuth

  • Widely used for scientific publishing


  • Product of Adobe Systems (Xerox spin-off 1984)

  • Programming language to create graphical output for printers

  • (fun to program)

  • Explicit support for fonts:

  • screen -- 70 dots per inch

  • laser printer -- 300+ dots per inch

  • type-setters -- 1,200 dots per inch

  • Other applications:

  • storage and transmission

  • window managers (News, NeXT)

Portable Document Format (PDF)

  • Adobe Systems -- based on experience with PostScript

  • Aims at storage and manipulation as well as rendering

  • Supports hyperlinks, searching, annotation, etc.

  • File sizes moderate except when they contain bit maps

  • Approximate fonts computed where necessary

PDF: Business Considerations

  • PDF is a proprietary format, but is a de facto standard:

  • Adobe publishes the specification

  • Adobe allows competitors to write PDF software

  • To stimulate use:

  • Adobe provides viewers for almost every type of computer

  • Adobe distributes the viewers without charge

  • Adobe makes revenue from the programs that create PDF files

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