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CSS: Cascading Style Sheets

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  1. CSS: Cascading Style Sheets

  2. What are Style Sheets • A style sheetis a mechanism that allows to specify how HTML (/XHTML/XML) pages should look • The style is specified by style rules • The style rules appear either in the document or in external files, called style sheets

  3. Style Sheets • Usually, a file that ends with .css • For example: • i.a.cnn.net/cnn/.element/ssi/css/1.1/main.css(CNN) • http://www.huji.ac.il/styles/style.css (HUJI) • To attach a style sheet to an HTML file, add <link rel="stylesheet"type="text/css"href="css-file"/>to the head

  4. Without a style sheet

  5. Simple Example <html> <head><title>A Joke</title></head> <body> <div><imgsrc="tomato.gif"alt="joke"/></div> <h1>A joke</h1> <p>A mama tomato, a papa tomato and a baby tomato are walking down the street. The baby tomato keeps falling behind so the papa tomato goes back, steps on the baby tomato and says, ketchup ("Catch-up!"). </p> </body> </html>

  6. Style File: joke.css body { background-image:url("bg.gif"); } h1 { background-color: green; color:rgb(250, 200, 250); /* pink */ font-family:cursive } p { background-color: yellow; color: purple; font-size:200%;}

  7. Simple Example (with css) <html> <head><title>A Joke</title> <linkrel="stylesheet"type="text/css"href="joke.css"/> </head> <body> <div><imgsrc="tomato.gif"alt="joke"></div> <h1>A joke</h1> <p>A mama tomato, a papa tomato and a baby tomato are walking down the street. The baby tomato keeps falling behind so the papa tomato goes back, steps on the baby tomato and says, ketchup ("Catch-up!"). </p></body> </html>

  8. Why do we Need a Style Sheet? • In XHTML (or HTML 4.01), styling is very limited, hence style sheets are necessary • Transitional versions are richer in style • Separates content from style • Reduces download time (how?) • Allows to easily maintain a consistent appearance over a whole Web site (how?)

  9. Declaration Selector Property Value(s) “important” rule Style Rules h1 { color:purple; font-family: Impact, Arial Black; font-size-adjust:.46; font-size:2.33em; font-weight:400; font-style:normal; text-decoration:none; word-spacing:normal !important; letter-spacing:normal; text-transform:none; }

  10. Style Rules (cont) • A rule has the following form selector {declaration block} • The selector determines when the rule is applied • For example, the following rule applies to text that is inside a <p> tag p{color: green;font-size:1.5em;font-style:italic}

  11. What Kind of Properties can a CSS Style Sheet Change? • Style properties • Layout properties • There are many properties and many possible values • We will not cover all of them here • Look in the Web !!! • A good source: http://www.w3schools.com/css

  12. Style Properties

  13. Our Examples • We use the following HTML example: This is<span>our example </span>for css. • The <span> tag is used to group elements for formatting with styles • Extremely useful tag...

  14. Font Properties • Font properties: family, size, weight, style, variant, ... span { font-family: courier; font-size:130%; font-style:italic; font-weight:bold}

  15. Text Properties • Text properties: color, transform, decoration, … span { color:#00cc00; text-decoration:line-through; text-transform:uppercase}

  16. Background Properties • Background properties: background-color, background-image, … span {background-color:#00ff00} span {background-image:url('bg.gif');}

  17. Layout Properties

  18. Page Layout • Each HTML element defines a layer (rectangular box) that is placed in some location on the page • Layers are nested with correspondence to the nesting of their elements

  19. Inline vs. Block Elements • There are two type of elements: • Block elements: p, ol, table, div, h1, etc. • Inline elements: b, i, a, span, cite, etc. • Layers of block elements are separated from their adjacent elements (i.e., a new line before and after), while inline elements are not • You can turn a block into an inline and vice-versa, using the display property, e.g., h1 { display: inline }

  20. Positioning Elements • Using CSS, you can define the position of an element inside its parent layer • For that, use the properties position, left, right, top and bottom span { position:relative; left:1cm; top: 1cm; color:#00cc00;}

  21. Position Types • But 1cm left to what?? • For that, we have the position property • Four position types: • static: the default position • absolute: relative to the parent layer coordinates • relative: relative to the static position • fixed: relative to the window coordinates

  22. Position Examples span { position:absolute; left:1cm; top: 1cm; color:#00cc00;} span { position:fixed; left:1cm; top: 1cm; color:#00cc00;}

  23. Position Examples span { position:static; left:1cm; top: 1cm; color:#00cc00;} Totally Ignored! This is thedefault position type

  24. More Layout Properties • Layer properties • margin-top (-bottom, -left, -right) • padding-top (-bottom, -left, -right) • border-width (-color, -style, … ) • Text Layout • direction, word-spacing, white-space, letter-spacing, line-height, text-align, text-indent, …

  25. Length Units • CSS has several types of length units: • em, ex: height of current fonts • px, in, cm, mm, pt, pc: international units • %: ratio of parent’s respective dimension • A page should remain a proper layout when windows (and even fonts) resize • Hence, do not assume anything about default sizes

  26. Selector Types

  27. Not supported by IE 5, 5.5 and 6 Several Kinds of Selectors • Type Selectors • Class Selectors • ID Selectors • Attribute Selectors • Universal Selector • Child Selectors • Adjacent-Sibling Selectors • Descendant Selectors • Pseudo-Class Selectors • Pseudo-Element Selectors

  28. Type Selector • A type selector is the name of an element type • A type selector matches every instance of the element type LI {color: red;font-size:16px} Matches: <ol> <li> An item </li> <liclass="reditem"> Another item </li> </ol>

  29. Class Selector • A class selector is a selector of the form x.y • It matches xs that have the classattribute with value y LI.reditem {color: red} .reditem { color: red} will also work! Matches: <ol> <li> An item </li> <liclass="reditem"> Another item </li> </ol>

  30. ID Selectors • IDs are identical to classes, except that there can only be one element with a given ID in a document LI#23 {color: red} #23 { color: red} will also work! Matches: <ol> <li> An item </li> <liid="23"> Another item </li> </ol>

  31. Attribute Selectors • p[attribute] • matches p when attribute is set to any value • p[title=intro] or p[title="intro"](the quotes are optional) • matches p when its title attribute is set to “intro” • p[class~=green] • matches p when the class attribute value includes the word “green”

  32. Universal Selector • The universal selector matches every element • The following rule means that all the text will have a size of 40px * {font-size: 40px }

  33. Descendant Selector • A descendant selector has the form S1 S2 where S1 and S2are selectors • It matches all elements that • match S2, and • are descendants (nested in) elements that match S1

  34. An Example p em {color: blue} Matches: This is <em>not blue</em>. <p> This is <em> blue </em> <span><i>and so is <em> this </em></i></span>. </p> What will this match? .href em {color: blue}

  35. Pseudo-Classes • Pseudo class selectors are similar to class selectors, but they match states rather than class values • For example, a link can be in the states: visited, active, mouse-over (“hover”), etc. • Another example: a text-area can be focused

  36. Examples of Rules for Pseudo-Classes • A:link {color: blue} • A:visited {color: purple} • A:hover {font-size: 1.5em} • A:active {color: red} • INPUT:focus {background-color: yellow}

  37. Grouping Selectors • We can group several declarations by specifying several selectors, separated by commas • For example, the following rule applies to all elements that match eitherH1,P B, orH2[class=largehead] P B, H1, H2.largehead{font-size: 120%}

  38. Inserting Style to a Page

  39. Inline Styles • In an inline style, the declaration block appear as the value of the attribute style of the element • Almost every tag can have the style attribute • exceptional: base, head, html, meta, param, script, style and title <pstyle="color: green; font-size: 1.5em; font-style: italic"> This text will be shown in italic green and the size will be 1.5 times the current font size </p>

  40. Document-Level Style <html> <head> <style type="text/css"> body{color: red;background: skyblue;} h1{color: blue } </style> </head> <body>... </body> </html>

  41. Imported Style Sheets • The @import rule imports the style rules of another style sheet • Usage: @import url(file.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

  42. Imported Style Sheets An Example: @importurl(general.css); body { color: red;background:skyblue } h1 {color: blue } Why do we need the import command?

  43. Inheritance and Cascading

  44. Inheritance of Properties • Consider a property of an element that does not match any rule • For some properties (inherited properties), the computed value of this property is inherited from the parent of the element • For example, color,font and word-spacing are inherited • Yet border, margin and padding are not!

  45. An Example • Given the rules: • body { font-size: 10pt } • h1 { font-size: 120% } • What will be the font size of the <em> element? Computed Value: 12pt <body> <h1>A <em>large</em> heading</h1> </body>

  46. Cascading of Styles • CSS merges style rules from different places (inline, document-level, linked and defaults) • Different places may have conflicting style rules • conflicts may even arise in a single source • The process of merging (cascading) styles from different places determines which style rules have higher priority

  47. Determining Property Values • Suppose that we would like to determine the value of property p for element e • Choose all declarations that have a selector that matches e and have the property p • Choose the first declaration in the cascading order, and apply the corresponding rule

  48. Cascading Order The cascading order of declarations: • Primary sort: importance of origin • Secondary sort: specificity of selectors • Final sort: order of appearance