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Cascading Style Sheets(CSS)
Part 2
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The Sources of Style Sheets
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Embedded Style Sheets
<HTML> <HEAD> <STYLE type="text/css"> <!-- BODY {color: red} --> </STYLE> </HEAD> <BODY> ... </BODY> </HTML>
The embedded style sheet appears in the header inside a <STYLE> element.
All the text of the style sheet is inside an HTML comment so that browsers that do not support CSS will ignore the text.
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Inline Styles
• Any tag, except the <HTML> tag itself, can have the style attribute
• For example, the following will define the color of the paragraph to be green
<P style="color: green"> and this is green</P>
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Imported Style Sheets
• The @import rule imports style rules to the beginning of the style sheet
• Usage: @import url(nameOfFile.css)• Several import rules may appear at
the beginning of the style sheet• Import rules can appear in embedded
style sheets or in external style sheets
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Linked Style Sheets
• Links to external style sheets can appear in the header of an HTML page
<HTML> <HEAD> <LINK rel="stylesheet" type="text/css”
href=“name.css“ media=“print handheld”> </HEAD> <BODY> ... </BODY></HTML>
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Inheritance and Cascading
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Inheritance of Properties
• If an element does not match any rule for some given property, then that property eitherhas an initial value, or inherits the value from the parent of the element
• The CSS2 Specification determines which properties are inherited, for example font-size is inheritedbackground-color is not inherited – its initial
value is transparent
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Computed Values are Inherited
• When a property is inherited, the computed value (and not a relative value) is inherited
• See the example on next the slide
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Example
• Given the rules:BODY { font-size: 10pt }H1 { font-size: 120% }
• What will be the font size of the EM element in the following HTML segment?
<BODY> <H1>A <EM>large</EM>
heading</H1></BODY>
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Cascading Order
• When several rules define the same property and match the same element, then the cascading order is as follows:Primary sort: weight and originSecondary sort: specificity of selectorsFinal sort: order of appearance
• The cascading order determines which rule applies
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Weight and Origin
• Author style sheets override user style sheets
• User style sheets override default (browser) style sheets
• ! important declaration overrides normal declaration
• If both author and user declarations have ! important, the user declarations still override the author declarations (in CSS2, but not in CSS1)
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Specificity of a Selector
• Let a be the number of ID attributes in the given selector
• Let b be the number of attributes and pseudo-classes in the given selector
• Let c be the number of element names in the given selector
• Sort the triples (a,b,c) lexicographically
• Pseudo-elements are ignored
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Example
• Consider the two rulesP {…}.introductory {…}
• If an element matches both, then the second rule has a higher specificity and overrides the first
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Order of Appearance
• If two rules have the same weight, origin and specificity, then the one that appears last in the style sheet overrides the others
• Rules in imported style sheets are considered to appear before local rules
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Lengths and Colors
How to Specify Them
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Colors
• Colors can be specified as namesAqua, Black, Blue, Fuchsia, Gray,
Green, Lime, Maroon, Navy, Olive, Purple, Red, Silver, Teal, White, Yellow
• Colors can be specified as #RRGGBBEach of RR, GG and BB should be a
hexadecimal number ranging from 00 to FF
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More Ways to Specify Colors
• RGB(rrr,ggg,bbb)Each of rrr, ggg and bbb should be a
decimal number ranging from 0 to 255
• RGB(rrr%,ggg%,bbb%)Each of rrr, ggg and bbb should be a
decimal number ranging from 0 to 100
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Lengths
• Lengths can be specified in absolute unitscm, in, mm, pc (6 picas per inch), pt (72
points per inch)• Length can be specified in relative
unitsem (the height of the current font), ex
(the height of the letter “x” in the current font), px (pixels – relative to the screen resolution and size)
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Using%
• Sometimes it is possible to use % to specify length (it is % of the current value of that property, as determined by inheritance and cascading)
• For examplefont-size: 200%
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The Box Model
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Elements as Boxes
• The element is the yellow area• The border is the thick black line• The padding is the area between the
element & the border• The margin is around the border, on the
outside (the dotted line is imaginary)
<p> This is text. </p>
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Don’t Be Mislead by the Diagram
• The padding has the same background color as the element
• The border can get its own color and style• The margin is transparent• Margins may collapse, i.e., adjoining margins of two
or more boxes may combine (overlap) to form a single margin
<p> This is text. </p>
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Padding Properties• The following properties determine the
width of the padding on each sidepadding-top padding-right padding-bottom padding-left
• The value isfixed length or % (of the width of the
containing box)• See examples in W3Schools
Alternatively, all four widths can be assigned to the property:
padding
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Border Properties
• The properties of the border areThe width (on each side)
•the possible values arethin, medium, thick, or length in some units (not %)
The border styleThe border color
• See examples in W3Schools
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Margin Properties
• The margin properties are the width on each side
• The value is one of the following:fixed length % (of the width of the containing box)Auto
• See examples in W3Schools
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Visual Formatting Model
Also Known as “Positioning”
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Inline-Level Elements
• Inline-level elements do not start a new line, for example<EM><SPAN>
• Inline-level elements generate inline boxes, which are positioned horizontally (i.e., side-by-side on the same line until the line is full and then continuing on the next line)
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Block-Level Elements
• Block-level elements start a new line, for example<P><LI><DIV>
• Block-level elements generate block boxes, which are positioned vertically (i.e., each box below the previous one)
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The Display Property
• The display property may force an element to become inline-level or block-level
• Some possible values of the display property areinlineblocknone
• See example in W3Schools
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Normal Flow
• Normal flow means that boxes are positioned left-to-right, top-to-bottom
• Each block box starts a new line
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Floats
• A floating box is first laid out according the normal flow,
andthen it is taken out of the normal flow
and shifted left or right as far as possible
• The normal flow continues along the side of a floating box
• See example in W3Schools
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Absolute Positioning
• The declaration position: absolute means that the box is taken out of the normal flow entirely and positioned in some fixed coordinates with respect to the containing block
• See example in W3Schools• More examples
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Relative Positioning
• The declaration position: relative means that the box is first laid out according to the normal flow and then it may be shifted relative to this position
• The next box is laid out as if the previous relative box was not shiftedRelative positioning may cause boxes to
overlap• See example in W3Schools
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The Clear Property
• The clear property is used to indicate that a box should not be placed next to the side of a float, but rather below that float
• The possible values are:none (the initial value)leftrightboth
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More Properties that Effect Positioning
• Clip• Overflow• Vertical-align• V-index• See examples in W3Schools
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Media Types
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Rules for Specific Media Types
• It is possible to define rules for specific media types
• See example in W3Schools• The visual formatting model may
behave differently for different media types
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Examples
A Three-Column Design
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Basic Idea
• Split the page into three column• Menu on the left• Main content in the center• Links (e.g., resources) on the right• Possibly some additional text is
positioned after the three columns
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The Rule for the Body
body{ background-color: gray; font-family: Tahoma, Verdana,
sans-serif; font-size: 400%; color: yellow; }
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The Rule for the Left Column
DIV#left{ float: left; width: 20%; height: 300px; background-color: #000066; }
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The Rule for the Center Column
DIV#center{ float: left; width: 60%; height: 350px; background-color: #660000; }
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The Rule for the Right Column
DIV.right{ float: left; width: 20%; height: 350px; background-color: #006600; }
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The Body
<div id="left"> <p> float: left; </p> </div><div id="center"> <p> float: left;
</p> </div><div class="right"> <p> float: left;
</p> </div>
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The Rule for More Text
<div style="position: static">More text goes here.……</div>
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In the Following Examples
• Sometimes the width is in px and sometimes the width is in %
• Sometimes the “More Text” DIV has clear: left
• In the following examples, we vary the value of the position property in each DIV
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The Examples
• Example 1: All positions are static• Example 2: All positions are
relative• Example 3: All DIVs are float: left• Example 4: DIV.right is float: right• Example 5: The DIV of the “More
Text” has clear: left
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Two Right Columns
• Example 6: DIV.right is float: right• Example 7: DIV.right is float: left
(and it also has margins)If the width of DIV.right is 20%, it will
never fit on the right side, because of the width of the margin
• Change the font size in the examples and see what happens
