8a. Cascading Style Sheet

Transcription

1 INFS 2150 Introduction to Web Development 8a. Cascading Style Sheet 1 Objectives Concepts of cascading style sheets (CSS). 3 ways of using CSS: inline styles, embedded styles, and external style sheet. Style precedence and style inheritance. Use cascading style sheets to format paragraphs, lists, and headings. Design a style for hypertext links in their four conditions. Define document content with the class and id attributes and create styles for them. Mark document content with the <div> and <span> tags and create styles for them. Use cascading styles to design page layout Peter Y. Wu, RMU 1

2 HTML and Page Layout Early versions of HTML had little support for Web page design. The philosophy was to: use basic text files that could be quickly downloaded rely on Web browsers to format the document s appearance The simplicity of HTML tags made creating Web pages easier and made pages load faster. 3 HTML and Page Layout Web authors looked for ways to deliver more visually interesting documents. This has been done chiefly in three ways: using HTML tag extensions converting text to images controlling page layout with tables Peter Y. Wu, RMU 2

3 HTML and Page Layout Upside HTML tag extensions: Provide the Web author with more choices in layout and design. Converting text to images: Don t have to worry whether a browser will support a particular font. Place in specific locations on the Web page. Tables: Use as a layout tool. 5 HTML and Page Layout Downside HTML tag extensions: Not all browsers support the various tag extensions. Converting text to images: You are limited in the number of inline images you can use and still have the page downloaded in a timely fashion. Moreover, it s difficult to make quick changes to the page s content if you have to edit the inline graphics first. Tables: This makes the HTML files more complicated to write and to interpret Peter Y. Wu, RMU 3

4 History and Support of CSS One principle of design theory is to separate the content of a document from its design. A style defines the appearance of a document element. The collection of styles for a Web page or Website is known as a style sheet. Style sheets use a common language and syntax. The main style sheet standard is Cascading Style Sheets (CSS). 7 Cascading Style Sheets (CSS) Like HTML, style sheets must use a common language and follow common rules. This language is known as Cascading Style Sheets, or more simply, CSS. CSS has been developed by the WWW Consortium ( the same organization that develops standards for HTML. CSS is designed to augment HTML, not replace it. CSS is a whole new way of formatting Web pages. CSS provides several tools not available with standard HTML Peter Y. Wu, RMU 4

5 CSS Versions CSS Level 1 was released in CSS Level 2 was released in CSS Level 3, the latest standard is being developed. Internet Explorer 6.0 and Mozilla Firefox 3.0 both support CSS Level 1, and most of CSS Level 2. CSS Level 3 support is uneven in many different browsers. Testing with different browsers is important. 9 Benefits of Style Sheets Use as a design tool. Makes website more flexible. Easier to maintain and modify. More aesthetically interesting. Consistent look Peter Y. Wu, RMU 5

6 Web Sites with Information on Cascading Style Sheets This figure displays websites that provide more information about the compliance of browsers with CSS1 and CSS2, and for information about the standards themselves. 11 Style Types There are three ways of employing CSS in Web pages: inline styles in which styles are added to each tag within the HTML file. The style affects that particular tag but does not affect other tags in the document. embedded styles applied to an entire HTML file, allowing the Web designer to modify the appearance of any tag in the document. linked or external style sheet placed in an external file and linked with pages in the Web site, allowing the Web designer to modify the appearance of tags in several documents Which approach you choose depends on the Web site s design Peter Y. Wu, RMU 6

7 Using Inline Styles If you need to format a single section in a Web page, you d probably use an inline style. To create in inline style, add the style attribute to the HTML tag using the following syntax: <tag style= style declarations > tag is the name of the tag (h1, h2, etc) style declarations are the styles you ll define for a particular tag style declaration must be enclosed within double quotation marks 13 A Style Declaration A style declaration consists of attribute names that specify such features as: font size color font type Attributes are followed by a colon and then the value of the attribute. Multiple attributes can be used as long as you separate each one by a semicolon. The general syntax for the style declaration is: attribute1:value1; attribute2:value2; Peter Y. Wu, RMU 7

8 Inserting an Inline Style This figure shows how to insert an inline style. 15 Heading with New Style This figure shows the heading with the new style Peter Y. Wu, RMU 8

9 Creating an Embedded Style An embedded style is a style applied to various sections within a Web page. Insert a <style> tag within the head section of the HTML file. Within the <style> tag, enclose the style declarations needed for the entire Web page. The syntax of an embedded style is: <style type= style sheet language > style declarations </style> style sheet language identifies the type of style language used in the document The default, is text/css for use with CSS 17 Selectors and Declarations Style declaration within the <style> tags obey the following syntax: selector {attribute1:value1; attribute2:value2;...} selector identifies an element in your document, such as a heading or paragraph, and the attributes and values within the curly braces indicate the styles applied to all the occurrences of that element this collection of attributes and values is also referred to as the declaration of the selector Peter Y. Wu, RMU 9

10 Selectors and Declarations For example, to display all h1 headings in the HTML document using a gold sans-serif font, use the following embedded style: <style> h1 {color: gold; font-family: sans-serif} </style> h1 is the selector and the text enclosed in the braces is the declaration 19 Defining a Global Style The type attribute was not included within the <style> tag. This is because text/css is the default style language, and unless you specify a different style language, you don t need to enter the type attribute. When using the <style> tags, you don t need to include double quotes around the attributes and attribute values Peter Y. Wu, RMU 10

11 Grouping Selectors You can apply the same declaration to a group of selectors by including all of the selector names separated by commas. The following is a sample style declaration to format all headings in a gold sans-serif font: <style> h1, h2, h3, h4, h5, h6 {color:gold; font-family:sans-serif} </style> 21 The Same Style Applied to Two Headings Even though the same style is used for all heading tags, there are still some differences in how the browser displayed text formatted with these tags. Most notably, the styles did not affect the relative sizes of the text. Text formatted with the <h1> tag is still larger than text formatted in the <h2> tag. This is because the size of the heading text was not defined, so that attribute is left to the browser Peter Y. Wu, RMU 11

12 Using an External Style Sheet Create styles that apply to an entire Web site by creating a text file containing style declarations. Most style sheets have the extension.css, though this is not a requirement. Within a style sheet, you don t need <style> tags, just the style declarations. 23 Linking to Style Sheets with the <link> Tag Use the <link> tag to link Web pages to a style sheet. The general syntax for using the <link> tag is as follows: <link href= URL rel= relation_type type= link_type > URL is the URL of the linked document relation_type establishes the relationship between the linked document and the Web page link_type indicates the language used in the linked document In order to link to a style sheet, the value of the rel attribute should be stylesheet and the value of the type attribute should be text/css. To link to a style sheet named mws.css, the <link> tag would be: <link href= mws.css rel= stylesheet type= text/css > Peter Y. Wu, RMU 12

13 Linking to Style Sheets Another way to link to a style sheet is to use command, which accesses the style sheet definitions from another file. To with styles, enclose command within the embedded <style> tags as follows: url(stylesheet.css); style declarations </style> stylesheet.css is the URL of the style sheet file 25 Command command provides greater flexibility than the <link> tag when working with multiple style sheets. command has limited browser support. Unless you have a compelling reason to you are probably better off using the <link> tag Peter Y. Wu, RMU 13

14 Resolving Style Precedence In cases where the styles conflict, precedence is determined in the following order: an inline style overrides any embedded style or external style sheet an embedded style overrides an external style sheet an external style sheet overrides the internal style rules set by the Web browser any style attributes left undefined by an inline style, an embedded style, or an external style sheet are left to the Web browser 27 Changing Styles As a change is made to a style at one level, the changes are cascaded through to the other levels (hence the term, cascading style sheets). Where a different font has not been specified, changes will cascaded through the embedded and inline styles. As you define more styles for a Web site, you need to keep track of the inline, embedded, and external style sheets to correctly predict the impact that style changes have on the appearance of each page Peter Y. Wu, RMU 14

15 Working with Style Inheritance Web pages invariably have elements placed within other elements. for example, a Web page might have a bold tag, <b>, placed within a paragraph tag, <p>, to create boldface text within the paragraph. The paragraph tag is likewise placed within the <body> tag. 29 Sample Tree Structure of HTML Elements This figure displays the HTML element relationship using a tree diagram Peter Y. Wu, RMU 15

16 Parent and Descendant Elements An element that lies within another element is called a descendant or descendant element. An element that contains another element is called the parent or parent element. An example of a parent is the <body> tag, which contains all of the other tags used to format the content of a Web page. Using the principle of inheritance, styles defined for each parent tag are transferred to its descendants. 31 Contextual Selectors Use the tree structure concept to better control how styles are applied to a Web page. CSS provides ways of fine-tuning the context in which the selector is applied. if you want to apply a style only to the descendant of a ancestral element, use the syntax: e1 e2 e1 and e2 are the names of HTML elements and e2 occurs below e1 in the hierarchy (tree structure) of elements in the document. for example: li b {color:blue} Color:blue is applied to <b> elements under <li> element Peter Y. Wu, RMU 16

17 Using Font Families The font-family attribute allows you to choose a font face for use in Web pages. CSS works with two types of font faces: a specific font, which is a font such as Arial, Garamond, or Times New Roman that is actually installed on a user s computer. a generic font, which is a general description of a font, allowing the operating system to determine which installed font best matches it. CSS supports five generic font types: serif, sansserif, monospace, cursive, and fantasy. 33 Generic Fonts This figure shows examples of each generic type. For each generic font there can be a wide range of designs Peter Y. Wu, RMU 17

18 Generic and Specific Fonts One issue with generic fonts is that you cannot be sure which specific font the Web browser uses to display your text. Whenever possible, it is a good idea to use specific fonts. Provide the Web browser with several fonts to choose from. Browsers that don t have access to the font you specified as your first choice may have your second or third choice available. List specific font names first, followed by a generic font name for the browser to use if none of the specific fonts can be found. 35 Managing Font Size The <address> tag can format address information on a Web page. by default, text formatted with the <address> tag is displayed in normal-sized type, italicized, and aligned with the left edge of the Web page. A common method for specifying font sizes with HTML is to use the size attribute of the <font> tag. The size attribute limits you to only seven font sizes. Browsers can display font sizes quite differently Peter Y. Wu, RMU 18

19 Managing Font Size Continued In CSS, you use the font-size attribute to manage font sizes. Font sizes can be expressed: as a unit of length with a keyword description as a percentage of the parent element with a keyword expressing the size relative to the font size of the parent element 37 Absolute and Relative Units If you choose to express size as a unit of length, you can use absolute units or relative units. absolute units define the font size based on one of the following standard units of measurement: mm (millimeter), cm (centimeter), in (inch), pt (point), and pc (pica). use a relative unit, which expresses the font size relative to a size of a standard character. There are two standard typesetting characters, referred to as em and ex. the em unit is equal to the width of the capital letter M in the browser s default font size the ex unit is equal to the height of a small x in the default font Peter Y. Wu, RMU 19

20 The EM and EX Units The em unit is more useful for page design, because 1 em is equal to the browser s default font size for body text. This is true no matter what font is being used (unlike the ex unit, whose size changes based on the font face being used). 39 The EM and EX Units As with absolute units, you can specify factional values for the em and ex units. Unlike the absolute units, em and ex units are scalable in that they retain their relative proportions regardless of the monitor size or resolution Peter Y. Wu, RMU 20

21 Pixels Pixels give you the greatest control over size. A pixel is the smallest element recognized by the monitor. Pixels should be used with some caution. Text that is 10 pixels high may be perfectly readable at a monitor resolution of 640 x 480, but it can become unreadable if the monitor is set to 1024 x Descriptive Keywords If you are uncomfortable dealing with units of length, you can use one of the seven descriptive keywords: xx-small x-small small medium large x-large xx-large These keywords correspond to the seven values of the size attribute in the <font> tag Peter Y. Wu, RMU 21

22 Expressing Font Size as a Percentage of the Parent Tag This figure shows the impact of such a style definition on boldface text in a Web page. The style has the same impact within a heading, since the heading is the parent element, and the boldface text is increased to 150% of the surrounding heading text. 43 Express a Font Size using Keywords Express a font size using the keywords larger and smaller, which makes the font one size larger or smaller than the size of the parent element. for example, to make the h2 heading one size larger than the body text, you could use the following style: body {font-size: medium} h2 {font-size: larger} Peter Y. Wu, RMU 22

23 Specifying Word, Letter, and Line Spacing Use CSS font attributes to control the spacing between letters, words, and lines of text. To set the space between individual letters, you use the letter-spacing attribute, with the syntax: letter-spacing: size size can either have the value normal, which allows the browser to determine the letter spacing based on the font being used, or a number expressed in the same measuring units used to describe font size (inches, millimeters, centimeters, em units, etc.) 45 Specifying Word, Letter, and Line Spacing Continued Another technique to change the spacing between individual words is the word-spacing attribute, with the syntax: word-spacing: size size is either equal to normal, to allow the browser to set the word spacing, or to a specific length using the standard units of measure Use the line-height attribute to modify the vertical space between lines of text. Graphic designers may know this spacing as leading. The line-height attribute specifies the minimum distance between the baselines of adjacent lines Peter Y. Wu, RMU 23

24 Comparison of Line Height to Font Size This figure shows how the line height relates to the font size. The line height is usually larger than the font size to leave additional space between lines of text. 47 Line Height To set the line height, use the style: line-height: size size is either a specific length, a percentage of the font size, or a number representing the ratio of the line height to the font size the standard ratio is 1.2, which means that the line height is 1.2 times the font size to make paragraphs double-spaced use the style definition p {line-height: 2} Peter Y. Wu, RMU 24

25 A Title with a Large Font Size and Small Line Height A common typographic technique is to create titles with large fonts and small line heights. This figure shows an example where the line height is actually smaller than the font size. This treatment can give the title greater impact than it would have with more space between the two lines. 49 Setting Font Styles and Weights Font styles are controlled by the font-style attribute. The font-style attribute has three possible values: normal, italic, or oblique. The italic and oblique styles are similar in appearance though there can be small differences depending on the font Peter Y. Wu, RMU 25

26 Font Weights CSS considers bold to be an aspect of the font s weight, or line thickness. Font weights can be expressed as an absolute number ranging in intervals of 100, going from 100 (the lightest) up to 900 (the heaviest or most bold ). For most fonts, you can assume that: a weight of 400 corresponds to normal text a weight of 700 can be used for bold text a weight of 900 for extra bold text 51 Font Weights Use the keywords normal and bold in place of a weight value. Express the font weight relative to the parent tag by using the keywords bolder or lighter. Use the CSS bolder attribute to get bolder text: h2 {font-weight: 700} b {font-weight: bolder} If these style definitions are applied to a Web page, h2 text formatted with the <b> tag will be bolder or thicker in appearance than the surrounding heading text Peter Y. Wu, RMU 26

27 Aligning Text Horizontally and Vertically Use the text-align attribute to left, center, right or justify text. To do this with CSS, use the text-align attribute: text-align: alignment alignment can be left, center, right, or justify setting the text-align value to justify stretches the text, extending it from the left to the right margin some browsers will ignore the text-align attribute value 53 Applying the Center Text-Align Style This figure shows how to apply the center text-align style Peter Y. Wu, RMU 27

28 Vertically Align Elements CSS allows you to vertically align elements such as text and images relative to the surrounding text. The syntax for setting the vertical alignment is: vertical-align: alignment alignment has one of the keyword values 55 Values of the Vertical Alignment Attribute This figure shows the alignment keyword values Peter Y. Wu, RMU 28

29 Examples of the Vertical Alignment Values This figure shows an example of each vertical-align value. Baseline is the default value for vertical alignment. 57 Indenting Text CSS allows you to indent the first line of a paragraph. The syntax for creating an indentation is: text-indent: indentation indentation is either the length, in either absolute or relative units, of the indentation or a percentage of the width of the paragraph Peter Y. Wu, RMU 29

30 Hanging Indent The length and percentage values also can be negative, which extends the first line to the left by the specified value or percentage, and then indents the rest of the lines in the paragraph. This particular effect, called a hanging indent, works sporadically on many browsers. 59 Special Text Attributes CSS provides three attributes for special text effects: text-decoration text-transform font-variant Peter Y. Wu, RMU 30

31 Values of the Text-Decorating Attribute This figure shows the text-decoration attribute can be used to underline your text or place a line over or through your text. You can also make your text blink on and off using the text-decoration: blink attribute. 61 The Text-Transform Attribute The text-transform attribute can be used to: capitalize the first letter of each word in a paragraph display the text in all capital letters display the text in all lowercase letters Peter Y. Wu, RMU 31

32 Values of the Text-Transform Attribute This figure shows the effect of the various text-transform values. 63 The Font-Variant Command Use the font-variant command to create small caps. Small caps are capital letters that are the same size as lowercase letters. The syntax for the font-variant attribute is: font-variant: small-caps Peter Y. Wu, RMU 32

33 Values of the Font-Variant Attribute This figure shows values of the font-variant attribute. 65 The font Attribute The font attribute provides an efficient way for you to define multiple attributes. The syntax for the font attribute is: font: font-style; font-variant; font-weight; font-size/lineheight; font-family font-style, font-variant, and so forth are the values for font and text style attributes The font attribute requires that you specify the font size, font variant, and font weight. If a font attribute is not included, the browser assigns the normal or standard value for the element Peter Y. Wu, RMU 33

34 The color Attribute CSS works with most of the color names supported by HTML. Another way to specify color in CSS is to use RGB color values. You can enter the hexadecimal form of the color value or the RGB color values directly. for example, to change the body text color to teal, use any of the following styles: body {color:teal} body {color: #008080} body {color: rgb (0,128,128)} body {color: rgb (0%, 50%, 50%} 67 Changing the Color of the H1-H6 Headings RGB color values range from 0 to 255, so specifying a color percentage of 50% for green and blue is close to a color value of 128. This figure shows an example of changing the color of the H1-H6 headings Peter Y. Wu, RMU 34

35 Working with Background Color By default, elements take on the background color of their parent element. To change the background color of almost any element, use the background-color style. 69 Applying a Background Color This figure shows how to apply a background color Peter Y. Wu, RMU 35

36 Working with Background Images Almost any element on the page can also be displayed with its own background image. The background image has four attributes: the source of the image file how the image is repeated in the background where the image is placed on the background whether the image scrolls with the display window To specify which file to use for a background, use the syntax: background-image: url(url) URL is the location of the image file 71 Applying a Background Image to an Element This figure demonstrates how you can apply this style to the <b> tag to create an interesting design for a section of boldface text Peter Y. Wu, RMU 36

37 Working with Background Images Continued By default, background images are tiled both horizontally and vertically behind the element until the entire element is filled. Control the way the tiling occurs using the background-repeat style attribute. The background-repeat attribute has four possible values. 73 Values of the Background-Repeat Attribute This figure shows the background-repeat attributes four possible values Peter Y. Wu, RMU 37

38 Examples of the Background-Repeat Values This figure shows examples of each background-repeat values. 75 The Background-Position Attribute Background images are placed in the upper-left corner of their element, and then repeated (if tiling is being used) from there. You can move the background image to a different location using the background-position style attribute. The background-position attribute has two values: the first indicates the distance from the left edge of the element the second indicates the distance from the element s top edge These values can be expressed as a percentage of the display area, in units of length, or with keywords Peter Y. Wu, RMU 38

39 Background-Position Keywords and Percentages This figure shows how background-position keywords relate to the percentage values. 77 The Background-Attachment Attribute By default, background images move along with the background of the page as the user scrolls through the Web page. To change the movement of background images, use the background-attachment attribute. The syntax of this style is: background-attachment: attach attached is either scroll, to scroll the image along with the element, or fixed, which places the image in a fixed place in the browser s display window, preventing it from moving even if the user scrolls down through the Web page Peter Y. Wu, RMU 39

40 Background Images Fixed background images are often used to create the impression of a watermark. a watermark is a term that refers to a translucent graphic impressed into the very fabric of the paper and used in specialized stationery If you use a background image that employs a transparent color, you can combine the background-color and background-image attributes to create a new image. for example, the style: body {background-color: yellow; background-image: url(logo.gif)} displays logo.gif on the background, and anywhere that a transparent color appears in the logo the background color yellow will shine through 79 The Background Attribute You can combine all of the various attributes for backgrounds into one attribute, called the background attribute. The syntax for the background attribute is: background: background-color background-image backgroundrepeat background-attachment background-position background-color, background-image, etc., are the values for the various background attributes Peter Y. Wu, RMU 40

41 Using the Background Style Attribute This figure shows an example of the background style attribute. 81 Working with List Styles CSS provides more control over the appearance and behavior of ordered, unordered, and definition lists than does HTML. CSS allows you to specify the types of labels attached to list items and how to position the labels with respect to the label text. The list-style-type attribute allows you to choose the type of label to display alongside text formatted with the <ul>, <ol>, or <li> tags Peter Y. Wu, RMU 41

42 Values of the List-Style-Type Attribute This figure shows the possible values of the list-style-type attribute. 83 Creating a Nested Outline Style Use contextual selectors to create an outline style for several levels of nested lists. This figure shows a set of contextual selectors used to create an outline style for different outline levels Peter Y. Wu, RMU 42

43 Using a list-style-image Attribute You can create a label, not included in the list-styletype values, with an image file and the liststyle-image attribute. The syntax for applying this attribute is: list-style-image: url(url) URL is the location and the filename of the image file It s a good idea to include the list-style-type attribute along with the list-style-image attribute. 85 Defining the List Style Position List items are treated by CSS as if they have an invisible box around them. The syntax for specifying the location of the list item label is: list-style-position: location location is either inside or the default value, outside Peter Y. Wu, RMU 43

44 Defining the Position of the List Label This figure shows that the labels for the list items can be placed either outside or inside the box. 87 The list-style Attribute You can combine all of these attributes into the list-style attribute. The syntax for this style is: list-style: list-style-type liststyle-image list-style-position list-style-type, list-style-image, and liststyle-position are the attribute values for each of the individual list style attributes Peter Y. Wu, RMU 44

45 Defining the Appearance of a List Label This figure shows how to define the appearance of a list label. 89 Formatting Hypertext Links Hypertext has an additional attribute that normal text doesn t have: the condition of the hypertext link itself. A hypertext link can be in one of four states: the link s target has already been visited by the user the link s target has never been visited by the user the link is currently being clicked by the user the user s mouse pointer is hovering over the link Web browsers provide a visual clue for each of these states, such as a different color for visited links, and a different shape for the pointer when it is hovering over a link Peter Y. Wu, RMU 45

46 Formatting Hypertext Links Continued CSS provides a different selector for each condition. The general syntax is: a:visited {styles for previously visited targets} a:link {styles for targets that have never been visited} a:active {styles for links that are currently being clicked} a:hover {styles when the mouse cursor is hovering over the link} - this is called a rollover effect You can use a variety of CSS attributes to create a different style for each condition. for example, to change the color of previously visited targets to red, use the style: a:visited {color:red} 91 Creating a Rollover Effect This figure shows the rollover effect Peter Y. Wu, RMU 46

47 Working with ids and Classes A pseudo-class is a classification of an element based on its status or its use. in the example of the rollover effect, the status was the condition of the hypertext link the element itself, a hypertext link with the pointer located over it, is called a pseudoelement CSS introduces additional pseudo-classes, including the first-line pseudo-class and the first-letter pseudoclass, which are used for formatting the first line and first letter of a block of text, respectively. 93 Applying a Style to a Pseudo-Class This figure shows how to apply a style to a pseudo-class and what it would like in the browser Peter Y. Wu, RMU 47

48 The class Attribute Many browsers do not support the first-letter and firstline pseudo-classes yet. The only pseudo-classes widely supported are the four hypertext link conditions. You can create customized classes by adding the class attribute to HTML tags. The syntax for creating a class is: <tag class= class_name > tag is the HTML tag class_name is the name of the class 95 Applying a Style to a Pseudo-Class This figure demonstrates creating an inline style for the h1 heading with the class name First Header. This technique is useful when you have multiple Web pages in which you want the first heading in each page to be formatted in the same way Peter Y. Wu, RMU 48

49 The id Attribute Closely related to the class attribute is the id attribute, which applies an id to a specific element in the document. The id attribute must be unique; there can not be more than one tag with the same id value. The syntax for creating an id is: <tag id= id_name > tag is the HTML tag id_name is an id name assigned to the tag The class and id attribute are useful HTML features to use with CSS to define styles for specific content without using inline styles. The id selector style rule can be applied only once per web page, The class selector style rule can be applied many times. 97 Creating a Class for Monthly Specials This figure shows how to create a class for monthly specials Peter Y. Wu, RMU 49

50 Working with Container Elements HTML supports two types of container types: the <span> tag, which is used to contain inline elements such as individual letters, words, phrases, or inline images the <div> tag, which is used to group blocks of text such as paragraphs, block quotes, headings, or lists. Collectively, these text blocks are known as block-level elements 99 Using the <div> Tag This figure shows an example in which a heading and a paragraph have been enclosed in a <div> container. The <div> tag does not actually format the block-level elements; it merely groups them as a unit. For this reason, the <div> tag always includes either a class or id attribute that identifies that group Peter Y. Wu, RMU 50

51 Using the <div> and <span> Tags This figure shows an example of how the <span> tag can be used to format a selection of text within a paragraph. A <div> tag is used to format the entire paragraph. You almost always include an id or class attribute with the <span> tag. 101 Formatting Block-Level Element Boxes With CSS, you can control the layout of a Web page by manipulating the size and location of block-level elements. CSS treats all block-level elements as a group. HTML tags that can be treated as block-level elements are: <h1> - <h6> tags <p> tag <blockquote> and <address> tags <ul>, <ol>, and <dl> list tags <li>, <dt>, or <dd> tags (individual list items) <div> tag <body> tag <hr> tag <img> tag Peter Y. Wu, RMU 51

52 Parts of the Block-Level Element Box There are three elements: margin between the box and the parent element border of the box padding, which is the space between the box around the block-level element and the border CSS provides attributes you can use to control the appearance and behavior of each of these elements. 103 Features of the Box Around a Block-Level Element This figure shows features of the box around a block-level element Peter Y. Wu, RMU 52

53 Some Block-Level Elements in a Web page. This figure shows some of the boxes in a Web page. 105 Controlling Margins The margin is the space between the block-level element and the parent element. There are four attributes that control the margin size: margin-top - the space between the top of the box and the top margin margin-right - the space between the right side of the box and the right margin margin-bottom - the space between the bottom of the box and the bottom margin margin-left - the space between the left side of the box and the left margin Peter Y. Wu, RMU 53

54 Controlling Margins Continued Margin sizes can be expressed in units of length (points, pixels, em units, etc.) or as a percentage of the width of the parent element box. Use the auto value, which allows the browser to determine the margin size. A margin size can be negative, although this can lead to unpredictable results when viewed with certain browsers. 107 Creating an Overlay Effect Web page designers can use negative margins to place one block-line element on top of another, creating an overlay effect. This figure shows an example of an overlay effect Peter Y. Wu, RMU 54

55 Controlling Margins The four margin attributes can be combined into a single attribute with the syntax: selector {margin-top margin-right margin-bottom margin-left} if you only include three values in the combined attribute, they are applied in the following order: top, right, bottom, and the browser sets the left margin to match the right margin if two values are specified, they are applied to the top and right margins, and the browser sets the bottom and left margins to match the top and right margins if only one value is entered, the browser applies the value to all four margins 109 Setting Padding Size Padding refers to the amount of space between the element and its border. Four attributes are used to control the size of the element s padding: padding-top padding-right padding-bottom padding-left Peter Y. Wu, RMU 55

56 Formatting the Border CSS provides a variety of attributes for managing the box s border width, border color, and border style. To combine all of the border attributes, use the syntax: border: border-width border-style border-color; These attributes can be applied to all four borders at once, or you can work with individual borders. There are a variety of ways with which border styles can be expressed. Support for the border declaration is inconsistent across browser types and versions. 111 Different Border Attributes The figure summarizes the various border attributes Peter Y. Wu, RMU 56

57 Example of Border-Style Values Border widths can be expressed using units of length or with the keywords thin, medium, or think. The border color can be defined using color names or color values. This figure shows that each of the nine different styles that can be applied to a border. 113 Formatting the Width and Height of Block-Level Boxes To change the width of a box, use the width attribute. Box width can be expressed in terms of absolute or relative units of length, or as a percentage of the width of the parent element. for example, the style: body {width: 75%} reduces the width of the Web page body to 75% of the width of the browser s display area The width attribute is seldom used except with text boxes and inline images Peter Y. Wu, RMU 57

58 Formatting the Width and Height of Block-Level Boxes Continued The height attribute sets the height of the element. Heights can be expressed in absolute or relative lengths, but not percentages. Typically, you won t set the height of a block-level element because problems can arise when the amount of text in the element exceeds the height allowed. The height attribute is usually applied to inline images and little else. 115 The float Attribute The float attribute works like the align= left or align= right attributes used with the <img> tags. This attribute places the block-level element on the left or right margin of the parent element Peter Y. Wu, RMU 58

59 Floating a Block-Level Element This figure shows that when a browser encounters the float attribute, it moves the element over to whatever margin the Web author has specified and then brings the next blocklevel element up. The text in that element is wrapped around the floating element. 117 Using the clear Attribute Prevent other elements from wrapping around the floating element by adding the clear attribute to the element below the floating element. When the value of the clear attribute is set to right, the browser displays the element on the page at the point where the right margin is clear. Other possible values for the clear attribute are left and both (for both margins) Peter Y. Wu, RMU 59

60 An Example Web site This figure shows an example Web site with the features discussed in this tutorial. 119 Summary The concepts of cascading style sheets. Three types of style sheets: inline, embedded or global, and linked or external style sheets. Syntax of CSS: declarations, selectors, attributes, and attribute values. Style precedence and style inheritance. HTML document structure: parent and descendant elements and contextual selectors. Color and background attributes. Font and text attributes including word and letter spacing, font styles and weights, and text alignment. Style attributes applied to lists. Pseudo-Class Selectors and Styles for Hyperlinks. Class Selectors and ID Selectors Use of <div> for block elements and <span> for in-line elements. Styles to control the margin, padding, and border attributes of their block-level elements Peter Y. Wu, RMU 60