Introduction
- 3.1 The CSS1 specification was developed in 1996
- CSS2 was released in 1998
- CSS3 is on its way
- CSSs provide the means to control and
change presentation of HTML documents
- CSS is not technically HTML, but can be
embedded in HTML documents
- A style sheet is a syntactic mechanism for
specifying style information
- Style sheets allow you to impose a standard
style on a whole document, or even a whole
collection of documents
- Style is specified for a tag by the values of its
properties
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3.2 Levels of Style Sheets
- There are three levels of style sheets
1. Inline - specified for a specific occurrence of a
tag and apply only to that tag
- This is fine-grain style, which defeats the
purpose of style sheets - uniform style
2. Document-level style sheets - apply to the
whole document in which they appear
3. External style sheets - can be applied to any
number of documents
- When more than one style sheet applies to a
specific tag in a document, the lowest level style
sheet has precedence
- In a sense, the browser searches for a style
property spec, starting with inline, until it finds
one (or there isnt one)
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3.2 Levels of Style Sheets (continued)
- Inline style sheets appear in the tag itself
- Document-level style sheets appear in the head of
the document
- External style sheets are in separate files,
potentially on any server on the Internet
- Written as text files with the MIME type text/css
- A <link> tag is used to specify that the browser
is to fetch and use an external style sheet file
<link rel = "stylesheet" type = "text/css"
href = "http://www.wherever.org/termpaper.css">
</link>
- An alternative way to reference an external style
sheet:
@import url(filename);
- Appears at the beginning of the content of a
style element (later)
- External style sheets can be validated
http://jigsaw.w3.org/css-validator/
validator-upload.html
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3.3 Style Specification Formats
- Format depends on the level of the style sheet
- Inline:
- Style sheet appears as the value of the style
attribute
- General form:
style = "property_1: value_1;
property_2: value_2;
property_n: value_n"
- Document-level:
- Style sheet appears as a list of rules that are the
content of a <style> tag
- The <style> tag must include the type attribute,
set to "text/css"
- Comments in the rule list must have a different
form - use C comments (/**/)
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3.3 Style Specification Formats (continued)
- General form:
<style type = "text/css">
rule list
</style>
- Form of the rules:
selector {list of property/values}
- Each property/value pair has the form:
property: value
- Pairs are separated by semicolons, just as in
the value of a <style> tag
- External style sheets
- Form is a list of style rules, as in the content of
a <style> tag for document-level style sheets
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3.4 Selector Forms
1. Simple Selector Forms
- The selector is a tag name or a list of tag names,
separated by commas
- Examples:
h1, h3
p
- Contextual selectors
ol ol li
2. Class Selectors
- Used to allow different occurrences of the same
tag to use different style specifications
- A style class has a name, which is attached to a
tag name
- For example,
p.narrow {property/value list}
p.wide {property/value list}
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3.4 Selector Forms (continued)
2. Class Selectors (continued)
- The class you want on a particular occurrence of
a tag is specified with the class attribute of the tag
- For example,
<p class = "narrow">
...
</p>
...
<p class = "wide">
...
</p>
3. Generic Selectors
- A generic class can be defined if you want a
style to apply to more than one kind of tag
- A generic class must be named, and the name
must begin with a period
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3.4 Selector Forms (continued)
3. Generic Selectors (continued)
- Example,
.sale { }
- Use it as if it were a normal style class
<h1 class = "sale"> Weekend Sale </h1>
...
<p class = "sale"> </p>
4. id Selectors
- An id selector allows the application of a style to
one specific element
- General form:
#specific-id {property-value list}
- Example:
#section14 {...}
5. Universal Selectors
* {color: red;}
- Applies to all elements in the document
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3.4 Selector Forms (continued)
6. Pseudo Classes
- Pseudo classes are styles that apply when
something happens, rather than because the
target element simply exists
- Names begin with colons
- hover classes apply when the mouse cursor is
over the element
- focus classes apply when an element has focus
<!-- pseudo.html -->
<html xmlns = "http://www.w3.org/1999/xhtml">
<head> <title> Checkboxes </title>
<style type = "text/css">
input:hover {background: pink;
color: red;}
input:focus {background: lightblue;
color: blue;}
</style>
</head>
<body>
<form action = "">
<p>
Your name:
<input type = "text" />
</p>
</form>
</body>
</html>
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3.5 Property Value Forms
- There are 60 different properties in 7 categories:
- Fonts
- Lists
- Alignment of text
- Margins
- Colors
- Backgrounds
- Borders
- Property Value Forms
- Keywords - left, small,
- Not case sensitive
- Length - numbers, maybe with decimal points
- Units:
px - pixels
in - inches
cm - centimeters
mm - millimeters
pt - points
pc - picas (12 points)
em - height of the letter m
ex - height of the letter x
- No space is allowed between the number
and the unit specification
e.g., 1.5 in is illegal!
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3.5 Property Value Forms
(continued)
- Percentage - just a number followed
immediately by a percent sign
- URL values
- url(protocol://server/pathname)
- Colors
- Color name
- rgb(n1, n2, n3)
- Numbers can be decimal or percentages
- Hex form: #XXXXXX
- Property values are inherited by all nested
tags, unless overriden
3.6 Font Properties
- font-family
- Value is a list of font names - browser uses
the first in the list it has
- font-family: Arial, Helvetica, Futura
- Generic fonts: serif, sans-serif, cursive,
fantasy, and monospace (defined in CSS)
- Browser has a specific font for each
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3.6 Font Properties (continued)
- If a font name has more than one word, it
should be single-quoted
- font-size
- Possible values: a length number or a name,
such as smaller, xx-large, etc.
- Font variants
- Default is normal, but can be set to small-caps
- font-style
- italic, oblique (useless), normal
- font-weight - degrees of boldness
- bolder, lighter, bold, normal
- Could specify as a multiple of 100 (100 900)
- font
- For specifying a list of font properties
font: bolder 14pt Arial Helvetica
- Order must be: style, weight, size, name(s)
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3.6 Font Properties (continued)
SHOW fonts.html and display
SHOW fonts2.html and styles.css and display
- The text-decoration property
- line-through, overline, underline, none
SHOW decoration.html & display
- letter-spacing value is any length property
value
3.7 List properties
- list-style-type
- Unordered lists
- Bullet can be a disc (default), a square, or a
circle
- Set it on either the <ul> or <li> tag
- On <ul>, it applies to all items in the list
<h3> Some Common Single-Engine Aircraft </h3>
<ul style "list-style-type: = square">
<li> Cessna Skyhawk </li>
<li> Beechcraft Bonanza </li>
<li> Piper Cherokee </li>
</ul>
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3.7 List properties (continued)
- On <li>, list-style-type applies to just that
item
<h3> Some Common Single-Engine Aircraft </h3>
<ul>
<li style = "list-style-type: disc">
Cessna Skyhawk </li>
<li style = "list-style-type: square">
Beechcraft Bonanza </li>
<li style = "list-style-type: circle">
Piper Cherokee </li>
</ul>
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3.7 List properties (continued)
- Could use an image for the bullets in an
unordered list
- Example:
<li style = "list-style-image:
url(bird.jpg)">
- On ordered lists - list-style-type can be used
to change the sequence values
Property value
decimal
upper-alpha
lower-alpha
upper-roman
lower-roman
Sequence type
First four
Arabic numerals
Uc letters
Lc letters
Uc Roman
Lc Roman
1, 2, 3, 4
A, B, C, D
a, b, c, d
I, II, III, IV
i, ii, iii, iv
SHOW sequence_types.html and display
- CSS2 has more, like lower-greek, and hebrew,
and armenian
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3.8 Colors
- Color is a problem for the Web for two reasons:
1. Old monitors vary widely
2. Old browsers vary widely
- There are three color collections
1. There is a set of 17 colors that are guaranteed
to be displayable by all graphical browsers on
all color monitors
black
navy
blue
green
teal
lime
aqua
maroon
orange
000000
000080
0000FF
008000
008080
00FF00
00FFFF
800000
FFA500
purple
olive
gray
silver
red
fuchia
yellow
white
800080
808000
808080
C0C0C0
FF0000
FF00FF
FFFF00
FFFFFF
- There are 140 named colors see Appx. B
2. There is a much larger set, the Web Palette
- 216 colors
- Use hex color values of 00, 33, 66, 99, CC,
and FF
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3.8 Colors (continued)
3. Any one of 16 million different colors
___________________________________________
- The color property specifies the foreground
color of elements
<style type = "text/css" >
th.red {color: red}
th.orange {color: orange}
</style>
<table border = "5">
<tr>
<th class = "red"> Apple </th>
<th class = "orange"> Orange </th>
<th class = "orange"> Screwdriver </th>
</tr>
</table>
- The background-color property specifies the
background color of elements
SHOW back_color.html and display
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3.9 Alignment of Text
- The text-indent property allows indentation
- Takes either a length or a % value
- The text-align property has the possible
values, left (the default), center, right, or
justify
- Sometimes we want text to flow around
another element - the float property
- The float property has the possible values,
left, right, and none (the default)
- If we have an element we want on the right,
with text flowing on its left, we use the
default text-align value (left) for the text and
the right value for float on the element we
want on the right
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3.9 Alignment of Text (continued)
<img src = "c210.jpg"
style = "float: right" />
-- Some text with the default alignment - left
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3.10 The Box Model
- Borders every element has a border-style
property
- Controls whether the element has a border and
if so, the style of the border
- border-style values: none, dotted, dashed,
and double
- border-width thin, medium (default), thick,
or a length value in pixels
- Border width can be specified for any of the
four borders (e.g., border-top-width)
- border-color any color
- Border color can be specified for any of the
four borders (e.g., border-top-color)
SHOW borders.html and display
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3.10 The Box Model (continued)
- Margin the space between the border of an
element and its neighbor element
- The margins around an element can be set with
margin-left, etc. - just assign them a length
value
<img src = "c210.jpg " style = "float: right;
margin-left: 0.35in;
margin-bottom: 0.35in" />
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3.10 The Box Model (continued)
- Padding the distance between the content of an
element and its border
- Controlled by padding, padding-left, etc.
SHOW marpads.html and display
3.11 Background Images
- The background-image property
SHOW back_image.html and display
- Repetition can be controlled
- background-repeat property
- Possible values: repeat (default), no-repeat,
repeat-x, or repeat-y
- background-position property
- Possible values: top, center, bottom, left,
or right
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3.12 The <span> and <div> tags
- One problem with the font properties is that
they apply to whole elements, which are
often too large
- Solution: a new tag to define an element in the
content of a larger element - <span>
- The default meaning of <span> is to leave the
content as it is
<p>
Now is the <span> best time </span> ever!
</p>
- Use <span> to apply a document style sheet to
its content
<style type = "text/css">
.bigred {font-size: 24pt;
font-family: Ariel; color: red}
</style>
...
<p>
Now is the
<span class = "bigred">
best time </span> ever!
</p>
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3.12 The <span> and <div> tags (continued)
- The <span> tag is similar to other HTML tags,
they can be nested and they have id and class
attributes
- Another tag that is useful for style specifications:
<div>
- Used to create document sections (or divisions)
for which style can be specified
- e.g., A section of five paragraphs for which
you want some particular style
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3.12 Conflict Resolution
- A conflict occurs when there are two or more
values for the same property on the same
element
- Sources of conflict:
1. Conflicting values between levels of style
sheets
2. Within one style sheet
3. Inheritance can cause conflicts
4. Property values can come from style sheets
written by the document author, the browser
user, and the browser defaults
- Resolution mechanisms:
1. Precedence rules for the different levels of
style sheets
2. Source of the property value
3. The specificity of the selector used to set the
property value
4. Property value specifications can be marked
to indicate their weight (importance)
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3.12 Conflict Resolution (continued)
- Weight is assigned to a property value by
attaching !important to the value
- Conflict resolution is a multistage process,
called the cascade:
1. Gather all of the style specs from the different
levels of style sheets
2. All available specs, from all sources, are sorted
by origin and weight, using the following rules,
which are given in precedence order:
a. Important declarations with user origin
b. Important declarations with author origin
c. Normal declarations with author origin
d. Normal declarations with user origin
e. Any declarations with browser (or other user
agent) origin
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3.12 Conflict Resolution (continued)
3. If any conflicts remain, sort them by
specificity:
a. id selectors
b. Class and pseudo-class selectors
c. Contextual selectors
d. Universal selectors
4. If there are still conflicts, resolve them by
precedence to the most recently seen
specification
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