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Cascading Style Sheets (CSS)
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Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides
Motivation
HTML markup can be used to represent
 Semantics: h1 means that an element is a top-level
heading
 Presentation: h1 elements look a certain way
It’s advisable to separate semantics from
presentation because:
 It’s easier to present documents on multiple platforms
(browser, cell phone, spoken, …)
 It’s easier to generate documents with consistent look
 Semantic and presentation changes can be made
independently of one another (division of labor)
 User control of presentation is facilitated
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Style Sheet Languages
Cascading Style Sheets (CSS)
 Applies to (X)HTML as well as XML documents
in general
 Focus of this chapter
Extensible Stylesheet Language (XSL)
 Often used to transform one XML document to
another form, but can also add style
 XSL Transformations covered in later chapter
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CSS Introduction
A styled HTML document
produced by the style sheet style1.css:
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CSS Introduction
link element associates style sheet with doc.
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CSS Introduction
type attribute specifies style language used
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CSS Introduction
href attribute provides style sheet URL
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CSS Introduction
title attribute provides style sheet name
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CSS Introduction
Alternative, user selectable style sheets
can be specified
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CSS Introduction
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CSS Introduction
A styled HTML document
produced by the style sheet style2.css:
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CSS Introduction
Note that alternate, user selectable style is
not widely supported: firefox 3 and IE 8 do,
but IE 6, IE 7 and Chrome don’t.
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CSS Introduction
Single document can be displayed on
multiple media platforms by tailoring style
sheets:
This document will be printed differently than
it is displayed.Css Founder.com

CSS Syntax
Parts of a style rule (or statement)
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CSS Syntax:
Selector Strings
Single element type:
Multiple element types:
All element types:
Specific elements by id:
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CSS Syntax:
Selector Strings
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CSS Syntax:
Selector Strings
Elements belonging to a style class:
 Referencing a style class in HTML:
Elements of a certain type and class:
class selector: begins with a period .
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CSS Syntax:
Selector Strings
this span belongs to three style classes
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CSS Syntax:
Selector Strings
this rule applies only to span’s belonging to class special
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CSS Syntax:
Selector Strings
Source anchor elements:
Element types that are descendents:
pseudo-classes
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CSS Syntax:
Selector Strings
Element types that are descendants:
rule applies to li element that is
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CSS Syntax:
Selector Strings
part of the content of an ol element
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CSS Syntax:
Selector Strings
part of the content of an ol element
that is part of the content of a ul element
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CSS Syntax
Style rules covered thus far follow ruleset
syntax
At-rule is a second type of rule
 Reads style rules from specified URL
 Must appear at beginning of style sheet
URL relative to style sheet URL
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Style Sheets and HTML
Style sheets referenced by link HTML
element are called external style sheets
Style sheets can be embedded directly in
HTML document using style element
Most HTML elements have style
attribute (value is list of style declarations)Css Founder.com

Style Sheets and HTML
Rules of thumb:
 Use external style sheets to define site-wide style
 Prefer style sheets (either external or embedded)
to style attributes
 XML special characters
 Must use references in embedded style sheets and
style attribute
 Must not use references in external style sheets
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CSS Rule Cascade
What if more than one style declaration
applies to a property of an element?
The CSS rule cascade determines which
style rule’s declaration applies
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CSS Rule Cascade
To find the value for an element/property
combination, user agents must apply the
following sorting order:
1- Find all declarations that apply to the
element and property in question, for the
target media type. Declarations apply if the
associated selector matches the element in
question.
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CSS Rule Cascade
2- The primary sort of the declarations is
by weight and origin: for normal
declarations, author style sheets override
user style sheets which override the default
style sheet. For "!important" declarations,
user style sheets override author style sheets
which override the default style sheet. "!
important" declaration override normal
declarations. An imported style sheet has
the same origin as the style sheet that
imported it.
Five origin/weight levels:
1. user/important
2. author/important
3. author/normal
4. user/normal
5. user agent/normal
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CSS Rule Cascade
3- The secondary sort is by specificity of selector: more
specific selectors will override more general ones. Pseudo-
elements and pseudo-classes are counted as normal elements
and classes, respectively.
Specificity:
1. style attribute
2. rule with selector:
1. ID
2. class/pseudo-class
3. descendant/element type
4. universal
3. HTML attribute
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CSS Rule Cascade
4- Finally, sort by order specified: if two rules have the
same weight, origin and specificity, the latter specified wins.
Rules in imported style sheets are considered to be before any
rules in the style sheet itself.
Conceptually, create one
long style sheet. Later
style rules have higher
priority than earlier rules.
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CSS Inheritance
What if no style declaration applies to a
property of an element?
Generally, the property value is inherited
from the nearest ancestor element that has a
value for the property
If no ancestor has a value (or the property
does not inherit) then CSS defines an initial
value that is used
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CSS Inheritance
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CSS Inheritance
Property values:
 Specified: value contained in declaration
 Absolute: value can be determined without reference
to context (e.g., 2cm)
 Relative: value depends on context (e.g., larger)
 Computed: absolute representation of relative
value (e.g., larger might be 1.2 x parent font
size)
 Actual: value actually used by browser (e.g.,
computed value might be rounded)
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CSS Inheritance
Most properties inherit computed value
 Exception discussed later: line-height
A little thought can usually tell you
whether a property inherits or not
 Example: height does not inherit
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CSS Font Properties
Glyph (visual representation)
character cell
(content area)
A font is a mapping from code points to glyphs
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CSS Font Properties
A font is a mapping from code points to glyphs
glyphs do not necessary stay inside cells!
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CSS Font Properties
A font family is a collection of related fonts
(typically differ in size, weight, etc.)
font-family property can accept a list of
families, including generic font families
first choice font
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CSS Font Properties
second choice font
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CSS Font Properties
generic
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CSS Font Properties
generic
fonts are
system-
specific
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CSS Font Properties
Note that most generic font can be easily
set on Firefox and Chrome, but such option
doesn’t seem to be available on IE 7 and 8. IE
will still default to something although maybe
not what you had hoped for!
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CSS Font Properties
Many properties, such as font-size, have a value that
is a CSS length
All CSS length values except 0 need units
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CSS Font Properties
Computed value
of font-size
property
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CSS Font Properties
Reference font defines em and ex units
 Normally, reference font is the font of the
element being styled
 Exception: Using em/ex to specify value for
font-size
parent element’s font is
reference font
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CSS Font Properties
Other ways to specify value for
font-size:
 Percentage (of parent font-size)
 Absolute size keyword: xx-small, x-small,
small, medium (initial value), large,
x-large, xx-large
 User agent specific; should differ by ~ 20%
 Relative size keyword: smaller, larger
 Relative to parent element’s font
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CSS Font Properties
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CSS Font Properties
Text is rendered using line boxes
Height of line box given by line-height
 Initial value: normal (i.e., cell height; relationship with
em height is font-specific)
 Other values (following are equivalent):
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CSS Font Properties
When line-height is greater than cell
height:
Inheritance of line-height:
 Specified value if normal or unit-less number
 Computed value otherwiseCss Founder.com

CSS Font Properties
font shortcut property:
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CSS Font Properties
Initial values used if no value specified in font
property list (that is, potentially reset)
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CSS Font Properties
specifying line-height (here, twice cell height)
any order size and family required,
order-dependent
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CSS Text Formatting
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CSS Text Color
Font color specified by color property
Two primary ways of specifying colors:
 Color name: black, gray, silver, white, red, lime,
blue, yellow, aqua, fuchsia, maroon, green, navy,
olive, teal, purple, full list at
http://www.w3.org/TR/SVG11/types.html#ColorKey
 red/green/blue (RGB) values
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CSS Text Color
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CSS Box Model
Every rendered element occupies a box:
(or inner edge)
(or outer edge)
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CSS Box Model
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