,

,

,

,

, and

. • PARENT represents any element that can be a valid parent of its children. • CHILD represents any element that can be a valid child of its parent. • LIST represents any list element including

,
, and

. • LIST_ITEM represents any list item including

,

, and

. • Selectors that you should replace are uppercase. If a selector contains lowercase text, that part of the selector should not be changed unless you also modify the HTML pattern, such as changing a class name. The following are typical placeholders: • SELECTOR {} represents any selector. • INLINE_SELECTOR {} represents any selector that selects inline elements. • INLINE_BLOCK_SELECTOR {} represents any selector that selects inline-block elements. • BLOCK_SELECTOR {} represents any selector that selects block elements. • TERMINAL_BLOCK_SELECTOR {} represents any selector that selects terminal block elements. • SIZED_BLOCK_SELECTOR {} represents any selector that selects sized block elements. • TABLE_SELECTOR {} represents any selector that selects table elements. • CELL_SELECTOR {} represents any selector that selects table cell elements. 8040FM.qxd 3/29/07 12:24 PM Page xxvi • PARENT_SELECTOR {} represents any selector that selects the parent in the design pattern. • SIBLING_SELECTOR {} represents any selector that selects the children in the pat- tern. • TYPE {} represents a selector that selects elements by a type of your choice such as h1 or span. • *.CLASS {} represents a selector that selects elements by a class name of your choice. • #ID {} represents a selector that selects elements by an ID of your choice. • Values that you should replace are represented by uppercase tokens. If a value contains lowercase text, that part of the value should not be changed. The following are typical value tokens: • Some values are literal and not meant to be replaced such as 0, -9999px, 1px, 1em, none, absolute, relative, and auto. These values are always lowercase. • +VALUE represents a positive measurement greater than or equal to zero, such as 0, 10px, or 2em. • -VALUE represents a positive measurement less than or equal to zero, such as 0, -10px, or -2em. • ±VALUE represents any measurement. • VALUEem represents an em measurement. • VALUEpx represents a pixel measurement. • VALUE% represents a percentage measurement. • VALUE_OR_PERCENT represents a value that can be a measurement or a percentage. • WIDTH STYLE COLOR represents multiple property values, such as those required by border. I use an uppercase token for each value. • url("FILE.EXT") represents a background image where you replace FILE.EXT with the URL of the image. • CONSTANT represents a valid constant value. For example, white-space allows three constant values: normal, pre, and nowrap. For convenience, I often list the valid constant values in uppercase with underscores in between each possible value, such as NORMAL_PRE_NOWRAP. • ABSOLUTE_FIXED represents a list of constant values from which you can choose one value. The underscore separates the constant values. The complete list of values for position includes static, relative, absolute, and fixed. If a design pattern only works for absolute and fixed, the pattern specifies position:ABSOLUTE_FIXED. If it works for all four values, it specifies position:STATIC_RELATIVE_ABSOLUTE_FIXED or position:CONSTANT. ■INTRODUCTION xxvii 8040FM.qxd 3/29/07 12:24 PM Page xxvii ■INTRODUCTIONxxviii • -(TAB_BOTTOM + EXTRA_BORDER + EXTRA_PADDING) is an example of a formula that you would replace with a calculated value. The uppercase tokens in the formula are tokens that occur elsewhere in the design pattern. For example, if you assigned TAB_BOTTOM to 10px, EXTRA_BORDER to 10px, and EXTRA_PADDING to 10px, you would replace the formula with -30px. Using This Book You can use the book to master CSS. You can read straight through the book to take your CSS skills to a higher level and to discover the many golden nuggets tucked away inside design patterns. Each chapter is organized so that it builds on design patterns presented earlier in the chapter and presented in previous chapters. On the other hand, since individual chapters and design patterns are self-contained, you can read them one by one in any sequence to master a specific topic or technique. You can use the book as a reference book. This book explains all of the usable CSS prop- erties and shows how to use them in examples. Even more importantly, many properties behave differently when combined with other properties. Each design pattern identifies and documents the unique combination of properties required to create a specific result. This makes it a reference book not only for how CSS properties work alone, but also for how they work in combination. You can use the book to learn by example. Since all examples in the book follow best practices, you can learn good habits and techniques just by studying them. To make studying the book by example easier, you can use the “See also” sections to look up all related design patterns. This allows you to easily see many examples of how a specific CSS property or fea- ture can be used in a variety of contexts. You can use the book as a cookbook to help you create designs or to solve problems. Design patterns are organized by topic so you can quickly find related solutions. I have added extra features to the book to make it easy to find a solution when you need it. You can use the table of contents, the index, thumb tabs, chapter outlines, design pattern names, and the “See also” section of each design pattern to quickly find properties, patterns, answers, and solutions. Since the screenshots in each example are in the same location on every page, you can even thumb through the book while looking at screenshots to find a solu- tion. I find visual scanning a very easy, fast, and effective way to find solutions! Companion Web Site The companion web site, cssDesignPatterns.com, is designed to enhance your experience with the book. Each page contains links to related design patterns and a search box for finding patterns. Each design pattern contains the example and its source code as well as additional information, such as additional examples, errata, comments, and links to related resources on the Internet. At the end of each design pattern in the book is a link to the design pattern on cssDesignPatterns.com. (Each design pattern on the web site is a directory named after the design pattern with spaces in the design pattern name replaced by hyphens.) In addition, cssDesignPatterns.com contains design patterns that are not in the book. 8040FM.qxd 3/29/07 12:24 PM Page xxviii How This Book Is Structured Chapters 1 through 3 explore the fundamentals of CSS and HTML: • Chapter 1 shows how design patterns make CSS easy. Here I demonstrate how to com- bine simple design patterns into more complex and powerful patterns. I also review the syntax of CSS and the cascade order. In addition, I present several charts that make using CSS easy: a list of links to useful CSS web sites, a one-page summary of CSS prop- erties; a four-page listing of all usable CSS properties, values, and selectors organized by where they can be used; charts on units of measure and font size; two example stylesheets for normalizing the styles of elements in all browsers; and a 12-step guide to troubleshooting CSS. • Chapter 2 introduces the design patterns that underlie HTML. In this chapter, I pres- ent the best practices of using HTML including coding in XHTML. I also explore the types of structures you can create with HTML including structural blocks, terminal blocks, multi-purpose blocks, and inlines. I also show how to use IDs and attributes for easy selection by CSS selectors. • Chapter 3 introduces design patterns for CSS selectors and inheritance. Here I demonstrate how selectors are the bridge between HTML and CSS. I present design patterns for type, class, ID, position, group, attribute, pseudo-element, pseudo-class, and subclass selectors. I also explore CSS inheritance. Chapters 4 through 6 explore the six CSS box models. They show how each HTML element is rendered as one of these six types of boxes (or not rendered at all). They demonstrate how the same properties produce different results in each box model, and how each box model flows differently from the other box models. • Chapter 4 explores the six box models: inline, inline-block, block, table, absolute, and float. • Chapter 5 explores the three ways of dimensioning a box: sized, shrinkwrapped, or stretched. • Chapter 6 explores each of the box model properties: margin, border, padding, background, overflow, visibility, and pagebreak. Chapters 7 through 9 explore how boxes flow or are positioned. • Chapter 7 explores the five positioning models (static, absolute, relative, fixed, and floated) and relates them to the six box models. • Chapter 8 explores the three ways a box can be positioned: for example, a box can be indented or outdented, offset from its siblings, or aligned and offset from its container. • Chapter 9 combines the patterns in Chapters 7 and 8: The combinations result in more than 50 design patterns for positioning elements—with a particular focus on absolute and fixed positioning. ■INTRODUCTION xxix 8040FM.qxd 3/29/07 12:24 PM Page xxix Chapters 10 through 12 explore in detail how inline boxes flow and how to style, space, and align text and objects: • Chapter 10 explores the properties that style text and also contains three design patterns for hiding text while remaining accessible to nonsighted users. • Chapter 11 shows how to space inline content horizontally and vertically. • Chapter 12 shows how to align inline content horizontally and vertically. Chapters 13 and 14 explore in detail how blocks and images flow and how they can be styled: • Chapter 13 explores blocks, starting with a discussion of the structural meaning of blocks and how you can visually display that meaning. It covers lists, inlining blocks, collapsed margins, run-in blocks, block spacing, and marginal blocks. • Chapter 14 explores images, such as image maps, semi-transparent images, replacing text with images, sprites, shadowed images, and rounded corners. Chapters 15 and 16 explore in detail how to style and lay out tables and cells. • Chapter 15 explores tables including table selectors, collapsed borders, hiding cells, vertically aligning content in cells, and displaying inline and block elements as tables. • Chapter 16 explores laying out table columns using 12 patterns, which automatically shrinkwrap columns, size them, proportionally distribute them, and so forth. Chapter 17 explores how the flow of floats can be used to create fluid layouts: • Chapter 17 shows how to create fluid layouts that automatically adapt to different devices, fonts, widths, and zoom factors. It also shows how to create interactive layouts using JavaScript. Chapters 18 through 20 show how to combine design patterns to create a variety of solutions to the same problem. Each solution addresses different needs and has different advantages and disadvantages. Besides being useful solutions in and of themselves, they demonstrate how you can combine patterns to solve any design problem. • Chapter 18 explores drop caps. Here I cover seven types of drop caps using seven different combinations of design patterns. • Chapter 19 explores callouts and quotes. The chapter demonstrates five types of callouts and three types of quotes. • Chapter 20 explores alerts. Here I present three types of interactive alerts and eight types of text alerts (i.e., attention getters). ■INTRODUCTIONxxx 8040FM.qxd 3/29/07 12:24 PM Page xxx Downloading the Code All code is available at www.cssDesignPatterns.com. You can also download the code at www.apress.com by searching for and going to the detail page for Pro CSS and HTML Design Patterns. On the book’s detail page is a link to the sample code compressed into a ZIP file. You can use a utility like WinZip to uncompress the code. Using the Code The code is arranged in folders, with a folder for each chapter. To make chapter folders easy to navigate, each folder name includes the chapter number and title. Inside each chapter folder are example folders: one for each design pattern presented in the chapter. So you can easily find examples, each example folder has the same name as its design pattern. This makes it easy and fast to find design patterns by searching folder names. Since the HTML in each example names and describes its design pattern, you can find a design pattern by searching for words inside HTML files. You could also search inside CSS files for examples that use a particular CSS property, such as display. To make it easy to view examples in multiple browsers, I put a file named index.html in the root folder that links to all design pattern folders. In turn, each folder contains a file named index.html that links to all the design patterns in that folder. These navigation pages make it quick to find and view each design pattern in each chapter. Each example folder contains all the files needed to make the example work. This makes it a breeze to use the examples in your own work: simply copy a folder and start making changes. You don’t have to worry about tracking down and including files from other folders. The most important files in each example folder are example.html and page.css. example.html contains the XHTML code for the example. page.css is the main stylesheet for the example. Each example also uses a CSS file named site.css. It contains a few nonessential font and heading rules that give all the examples in the book the same basic look and feel. In a few exceptional cases, I use an additional CSS file to overcome bugs or nonstandard behavior in Internet Explorer.4 ie6.css contains rules to fix problems in Internet Explorer 6. ie7.css contains rules to fix problems in Internet Explorer 7. ie67.css contains rules to fix problems in both versions 6 and 7. Rules in these files override rules in page.css. The seven JavaScript examples use five JavaScript files. These are explained in the Event Styling design pattern Chapter 17. page.js is the most important file because it contains JavaScript code specific to the example. The remaining JavaScript files are open source libraries. Lastly, each example folder contains all image files used by that example. ■INTRODUCTION xxxi 4. There are only 25 of these files out of more than 350 design patterns. Most of these files contain only a single, simple rule, such as div{zoom:1;}. In spite of the numerous bugs, quirks, and nonstandard features of Internet Explorer 6, I only needed to build workarounds into 25 design patterns. This is because I carefully designed the patterns in this book to avoid problems in the first place. I allowed an exception in a pattern only when I could find no alternative. I literally had to throw out hundreds of design patterns to find patterns that work without exception. Lastly, because Internet Explorer 7 fixes most of the bugs in Internet Explorer 6, only 4 of these 25 exceptions apply to Internet Explorer 7. 8040FM.qxd 3/29/07 12:24 PM Page xxxi Errata You can view errata at www.cssDesignPatterns.com and on the detail page of the book at www.apress.com. If you find an error in the book, I would greatly appreciate knowing about it. Please e-mail the problem to support@apress.com and support@cssDesignPatterns.com. Contacting the Author You can contact me at mike@cssDesignPatterns.com. I look forward to your comments, sugges- tions, and questions. ■INTRODUCTIONxxxii 8040FM.qxd 3/29/07 12:24 PM Page xxxii Design Patterns: Making CSS Easy! On the surface, CSS seems easy. It has 45 commonly used properties you can employ to style a document. Below the surface, different combinations of properties and property values trig- ger completely different results. I call this CSS polymorphism because the same property has many meanings. The result of CSS polymorphism is a combinatorial explosion of possibilities. Learning CSS is more than learning about individual properties. It is about learning the contexts in which properties can be used and how different types of property values work dif- ferently in each context. As an example, take the width property, which has many different meanings depending on how it is combined with other rules and what values are assigned to it. For instance, width has absolutely no effect on inlines. width:auto shrinkwraps floats to the width of their content. width:auto shrinkwraps absolutes when left and right are set to auto. width:auto stretches blocks to the width of their parent element. width:auto stretches absolutes to the width of their containing block when left and right are set to 0. width:100% stretches blocks and floats to the width of their parent element as long as they do not have borders, padding, and margins. width:100% stretches tables to the width of their parent even if they do have borders and padding. width:100% stretches absolutes to the width of their closest positioned ancestor instead of their parent. width:100em sizes an element in relation to the height of its font-size, which allows the element to be sized wide enough to contain a certain number of characters. width:100px sizes an element to a fixed number of pixels regardless of the font-size of its text. To complicate matters further, not all of the rules are implemented by browsers. For example, over 40 out of 122 properties and over 250 out of 600 CSS rules are not implemented by one or more of the major browsers. Trying to learn CSS by memorizing the extraordinary number of exceptions to each rule is extremely frustrating. To make learning CSS easy, this book documents all usable combinations of properties and property values. It puts properties in context and paints a complete picture of how CSS works. Imagine the time you will save by not having to read about rules that do not work and by not having to test every rule to see whether it works in every browser and in combination with other rules. I have already done this for you. I have run many thousands of tests. I have tested every CSS property and every combination of properties in every major browser including Internet Explorer 7, Internet Explorer 6, Firefox 2, Opera 9, and Safari 2. 1 CHAPTER 1 8040CH01.qxd 3/22/07 10:35 AM Page 1 I have boiled down these results into 350+ simple design patterns—all the CSS and HTML design patterns you need to create stunning, high-performance, and accessible web sites. After you learn these design patterns, you’ll wonder how you ever developed web sites without them! In this chapter, I discuss the purpose of design patterns and how they work. I give some examples of how to combine design patterns to create new patterns. I also discuss how to use stylesheets, CSS syntax, and the cascading order to your advantage. Next, I present a series of charts that list all the usable CSS properties and units of measure. I then present 12 techniques for troubleshooting CSS quickly. Lastly, I discuss how to stan- dardize the way various browsers style elements—so you can override these default styles with confidence. Design Patterns—Structured Recipes Design patterns have been used with great success in software programming. They improve productivity, creativity, and efficiency in web design and development, and they reduce code bloat and complexity. In the context of CSS and HTML, design patterns are sets of common functionality that work across various browsers and screen readers, without sacrificing design values or accessibility or relying on hacks and filters. But until now they have not been applied systematically to HTML and CSS web design and development. Design patterns underlie all creative activities. We think in terms of patterns when we talk, write, and create. Design patterns are similar to document templates that we can fill in with our own content. In literature, they are like archetypal characters and plots. In music, they are like themes and variations. In programming, they are similar to reusable algorithms that can be systematically varied and combined with each other to produce a desired result. Once a design pattern is revealed, it greatly increases creativity and productivity. It can be used by itself to create quick results, and it can be easily combined with other patterns to create more complex results. Design patterns simplify and amplify the creative process. They make creation as easy as building with blocks or Legos. You simply choose predesigned pat- terns, vary them, and combine them to create the result you want. Patterns do not limit creativity—they unleash creativity. The seminal work Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (Addison-Wesley, 1995) explains that a design pattern consists of four elements: a pattern name, a problem, a solution, and tradeoffs. This book follows this approach. Since this is a practical book, it focuses directly on the concrete patterns designed into CSS and HTML that are actually implemented in the major browsers. This book also creates new design patterns by combining built-in patterns into higher-level patterns. In a very real sense, this is a book of patterns that you can use to create your designs. Using Design Patterns Chapters 1 through 7 present the basic properties and elements for styling layout. Chapters 8 and 9 combine these properties to create all possible block, positioned, and floated layouts. Chapters 10 through 12 present the basic properties for styling text and also present combina- tions of properties you can use to create inline layouts. Chapters 13 through 16 combine CHAPTER 1 ■ DESIGN PATTERNS: MAKING CSS EASY!2 M A K I N G C S S E A S Y ! 8040CH01.qxd 3/22/07 10:35 AM Page 2 design patterns from previous chapters with specialty properties and elements to style blocks, lists, images, tables, and table columns. Together, Chapters 1 through 16 present over 300 design patterns created by combining 45 common CSS properties with four types of elements (inline, inline-block, block, and table) and five types of positioning (static, relative, absolute, fixed, and float). This is the great power of design patterns: it is easy to take basic patterns and combine them to form more complex patterns. This makes learning CSS easy, and it makes using CSS very productive. Chapters 17 through 20 show how to combine these design patterns to create fluid layouts, drop caps, callouts, quotes, and alerts. To illustrate the simplicity and power of design patterns, the next five examples show how to take a series of basic design patterns and combine them into more complex patterns. You do not need to understand the details of each pattern—just the process of combining pat- terns. The first example in this series shows the background property in action. background is a design pattern built into CSS that displays an image behind an element. Example 1-1 shows the background property combined with a division element. The division is sized 250 by 76 pixels so it will reveal the entire background image.1 Example 1-1. Background Image HTML

Background Image

CSS div { background:url("heading2.jpg") no-repeat; width:250px; height:76px; } Example 1-2 demonstrates the Absolute design pattern. The idea behind the Absolute design pattern is to remove an element from the flow and position it relative to another CHAPTER 1 ■ DESIGN PATTERNS: MAKING CSS EASY! 3 M A K I N G C S S E A S Y ! 1. This example is simple and yet it combines seven design patterns: the Structural Block Elements design pattern in Chapter 2; the Type Selector pattern in Chapter 3; the Block Box pattern in Chap- ter 4; the Width, Height, and Sized patterns in Chapter 5; and the Background design pattern in Chapter 6. 8040CH01.qxd 3/22/07 10:35 AM Page 3 element. CSS provides the position:absolute rule for this purpose. When position:absolute is combined with the top and left properties, you can position an element at an offset from the top left of its closest positioned ancestor. I used position:relative to position the division so it would be the closest positioned ancestor to the span. I then absolutely positioned the span 10 pixels from the top and left sides of the division.2 Example 1-2. Absolute HTML

Absolute

Sized Absolute

CSS *.positioned { position:relative; } *.absolute { position:absolute; top:10px; left:10px; } /* Nonessential styles are not shown */ Example 1-3 combines the design patterns in the first two examples to create the Text Replacement design pattern. The idea behind text replacement is to display an image in the place of some text (so you can have more stylistic control over the text because it is embedded in an image). In addition, you want the text to be present behind the image so that it becomes visible if the image fails to download. CHAPTER 1 ■ DESIGN PATTERNS: MAKING CSS EASY!4 M A K I N G C S S E A S Y ! 2. This example is simple, and yet it combines seven design patterns: the Inline Elements and Structural Block Elements design patterns in Chapter 2; the Class Selector pattern in Chapter 3; the Absolute Box pattern in Chapter 4; and the Absolute, Relative, and the Closest Positioned Ancestor patterns in Chapter 7. 8040CH01.qxd 3/22/07 10:35 AM Page 4 I combined the Background and Absolute design patterns to create the Text Replacement pattern. I placed an empty span inside a heading. I relatively positioned the heading so child elements can be absolutely positioned relative to it. I assigned a background image to the span and absolutely positioned it in front of the text in the heading element. I sized the span and the heading to the exact size of the background image. The end result is that the background image of the span covers the text in the heading, and if the image fails to download, the styled text in the heading is revealed.3 Example 1-3. Text Replacement HTML

Text Replacement

Heading 2

CSS #h2 { position:relative; width:250px; height:76px; overflow:hidden; } #h2 span { position:absolute; width:250px; height:76px; left:0; top:0; background:url("heading2.jpg") no-repeat; } Example 1-4 demonstrates the Left Marginal design pattern. The idea behind this pattern is to move one or more elements out of a block into its left margin so you can have headings (or notes, images, etc.) on the left and content on the right.4 CHAPTER 1 ■ DESIGN PATTERNS: MAKING CSS EASY! 5 M A K I N G C S S E A S Y ! 3. The Text Replacement example uses the 14 design patterns shown in the previous two examples. It also introduces the ID Selector design pattern in Chapter 3. You can learn more about the Text Replacement design pattern in Chapter 10. 4. The Left Marginal design pattern combines the Position Selector design pattern in Chapter 3; the Margin pattern in Chapter 6; the Absolute Box pattern in Chapter 4; and the Absolute, Relative, and the Closest Positioned Ancestor patterns in Chapter 7. 8040CH01.qxd 3/22/07 10:35 AM Page 5 Example 1-4. Left Marginal HTML

Left Marginal

Heading

You want to excerpt an element and move it into the left margin.

CSS *.left-marginal { position:relative; margin-left:200px; } *.marginal-heading { position:absolute; left:-200px; top:0; margin:0; } Example 1-5 demonstrates the Marginal Graphic Dropcap design pattern. This pattern combines all the design patterns shown in the previous four examples. The idea behind this pattern is to create a graphical drop cap in the left margin of a block with all the advantages of the Text Replacement and Left Marginal design patterns.5 To meet these requirements, I used the indent class to relatively position the paragraph so that it will be the closest positioned ancestor of the drop cap and to add a 120-pixel left mar- gin to the paragraph to make room for the drop cap. I used the graphic-dropcap class to absolutely position the drop cap, to move it into the paragraph’s left margin, and to set it to the exact size of the dropcap image. I then absolutely positioned the span inside the graphic drop cap and moved it over the dropcap text so it covers the text with its background image. Viewed by itself, the Marginal Graphic Dropcap pattern is a somewhat complex combina- tion of 16+ design patterns. On the other hand, when viewed as a combination of the Text Replacement and Left Marginal design patterns, it is quite simple. This is the power of design patterns. CHAPTER 1 ■ DESIGN PATTERNS: MAKING CSS EASY!6 M A K I N G C S S E A S Y ! 5. The Marginal Graphic Dropcap design pattern is discussed in detail in Chapter 18. 8040CH01.qxd 3/22/07 10:35 AM Page 6 Example 1-5. Marginal Graphic Dropcap HTML

Marginal Graphic Dropcap

Marginal Graphic Dropcap. The letter M has been covered by the dropcap image. Screen readers read the text and visual users see the image. If the browser cannot display the dropcap image, the text becomes visible.

CSS *.indent { position:relative; margin-left:120px; } *.graphic-dropcap { position:absolute; width:120px; height:90px; left:-120px; top:0; } *.graphic-dropcap span { position:absolute; width:120px; height:90px; margin:0; left:0; top:0; background:url("m.jpg") no-repeat; } Using Stylesheets You can place styles in three locations: stylesheets,

CSS syntax is EASY!

CSS body { font-family:"Century Gothic",verdana,arial,sans-serif; font-size:20px; line-height:150%; margin:1em; border:4px double black; padding:0.25em; background-image:url("gradient.gif"); background-repeat:repeat-x; } p { margin:0; } span { font-weight:900; } CHAPTER 1 ■ DESIGN PATTERNS: MAKING CSS EASY! 13 M A K I N G C S S E A S Y ! 8040CH01.qxd 3/22/07 10:35 AM Page 13 Using Cascade Order CSS allows you to assign the same rule to the same element multiple times. I call these com- peting rules. Browsers use the cascading order to determine which rule in a set of competing rules gets applied. For example, a browser assigns default rules to each element. When you assign a rule to an element, your rule competes with the default rule, but since it has a higher cascading priority, it overrides the default rule. The cascading order divides rules into six groups based on the type of selector used in the rule. A rule in a higher-priority group overrides a competing rule in a lower-priority group. Groups are organized by the specificity of their selectors. Selectors in lower-priority groups have less specificity than selectors in higher-priority groups. The guiding principle behind the cascade order is that general selectors set overall styles for a document and more specific selectors override the general selectors to apply specific styles. For example, you may want to style all elements in a document with no bottom margin using *{margin-bottom:0;}. You may also want to style all paragraphs in a document with a bottom margin of 10 pixels using p{margin-bottom:10px;}. You may also want to style the few paragraphs belonging to the double-space class with a bottom margin of 2 ems using *.double-space{margin-bottom:2em;}. You may also want to style one paragraph with an extra-large bottom margin of 40 pixels using #paragraph3{margin-bottom:40px;}. In each of these cases, the cascade order ensures a more specific selector overrides a more general one. Following are the six selector groups listed from highest to lowest priority: 1. The highest-priority group contains rules with !important added to them. They override all non-!important rules. For example, #i100{border:6px solid black!important;} takes priority over #i100{border:6px solid black;}. 2. The second-highest-priority group contains rules embedded in the style attribute. Since using the style attribute creates hard-to-maintain code, I do not recommend using it. 3. The third-highest-priority group contains rules that have one or more ID selectors. For example, #i100{border:6px solid black;} takes priority over *.c10{border:4px solid black;}. 4. The fourth-highest-priority group contains rules that have one or more class, attribute, or pseudo selectors. For example, *.c10{border:4px solid black;} takes priority over div{border:2px solid black;}. 5. The fifth-highest-priority group contains rules that have one or more element selec- tors. For example, div{border:2px solid black;} takes priority over *{border:0px solid black;}. 6. The lowest-priority group contains rules that have only a universal selector—for example, *{border:0px solid black;}. When competing rules belong to the same selector group (such as both rules contain ID selectors), the type and number of selectors prioritize them further. A selector has higher priority when it has more selectors of a higher priority than a competing selector. For example, CHAPTER 1 ■ DESIGN PATTERNS: MAKING CSS EASY!14 M A K I N G C S S E A S Y ! 8040CH01.qxd 3/22/07 10:35 AM Page 14 #i100 *.c20 *.c10{} has a higher priority than #i100 *.c10 div p span em{}. Since both selectors contain an ID selector, they are both in the third-highest-priority group. Since the first has two class selectors and the second has only one class selector, the first has higher priority—even though the second has more selectors. When competing rules are in the same selector group and have the same number and level of selectors, they are further prioritized by location. Any rule in a higher-priority location overrides a competing rule in a lower-priority location. (Again, this only applies when com- peting rules are in the same selector group and have the same number and level of selectors. Selector groups always take precedence over location groups.) The six locations are listed here from highest to lowest priority: 1. The highest-priority location is the

Header Elements

CHAPTER 2 ■ HTML DESIGN PATTERNS42 H T M L D E S I G N P A T T E R N S 8040CH02.qxd 3/2/07 10:24 AM Page 42 Header Elements Problem You want to add metadata to a document. You also want to link the document to stylesheets and JavaScript files. You also want to improve performance by embedding CSS rules and JavaScript inside the page. Solution You can use to link stylesheets to a document. You can use href="URI" to specify the URI of the stylesheet. You can use media="all" to apply a stylesheet to all devices. You can use media="print" to apply a stylesheet only when printing. This allows you to hide navigational bars, remove backgrounds, reset inverse color schemes (like white text on a black background) to normal black text on a white background, and so forth. You can use media="handheld" to apply a stylesheet to handheld devices only. You may find this impractical because styles that work on one handheld device may be ignored or not work at all on another. Few browsers have implemented the following media types: "tty", "tv", "projection", "braille", and "aural". You can use to provide a user with alternate stylesheets. Browsers like Firefox 2 and Opera 9 put alternate style- sheets in a drop-down list and allow users to select and apply one alternate stylesheet at a time to a document. Since most web sites do not provide alternate stylesheets and since there is no visual indication that they are available, few users look for them or use them. Thus, sites that supply alternate stylesheets often put buttons or menus in the document and link them to JavaScript that switches between alternate stylesheets. You can embed styles in the Location ,