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Web Programming with HTML, XHTML, and CSS Second Edition- P5

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Web Programming with HTML, XHTML, and CSS Second Edition- P5: Điều này bắt đầu hướng dẫn nhận xét HTML và cũng giới thiệu đến bạn bằng cách sử dụng XHTML cho cấu trúc của một trang web và cascading style sheets (CSS) để kiểm soát như thế nào một tài liệu sẽ xuất hiện trên một trang web. Bạn se tìm hiểu làm thế nào để tận dụng lợi thế của các tính năng mới của các trình duyệt trong khi đảm bảo rằng các trang của bạn vẫn làm việc ở cũ, nhưng phổ biến, các trình duyệt. Bằng...

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  1. Chapter 5: Forms Figure 5-20 Str ucturing Your Forms with and Elements Large forms can be confusing for users, so it’s good practice to group together related form controls. The and elements do exactly this — help you group controls. Both elements were introduced in IE 4 and Netscape 6; however, older browsers will just ignore these elements, so you are safe to include them in all your forms. ❑ The element creates a border around the group of form controls to show that they are related. ❑ The element allows you to specify a caption for the element, which acts as a title for the group of form controls. When used, the element should always be the first child of the element. In the following example, you can see how a form has been divided into four sections: contact information, competition question, tiebreaker question, and enter the competition (ch05_eg17.html). Contact Information First name: Last name: E-mail: Competition Question How tall is the Eiffel Tower in Paris, France? 584ft 784ft 984ft 1184ft Tiebreaker Question 171
  2. Chapter 5: Forms In 25 words or less, say why you would like to win $10,000: Enter competition You can see how the elements create borders around the groups of form controls, and how the elements are used to title the groups of controls. Remember that the element must be the first child of the element when it is used. See Figure 5-21. Figure 5-21 The element can take the following attributes: ❑ All the universal attributes ❑ The basic event attributes Note that if you use a table to format your form, the element must appear inside the element. If a resides within a table that is used to format the page, then the entire fieldset must reside within the same cell. 172
  3. Chapter 5: Forms The element can take the following attributes: ❑ accesskey, which you will learn about in the next section ❑ align (which is deprecated — you should use CSS positioning instead) ❑ All of the universal attributes ❑ UI event attributes Focus When a web page featuring several links or several form controls loads, you might have noticed that you are able to use your Tab key to move between those elements (or Shift+Tab to move backward through elements). As you move between them, the web browser tends to add some form of border or highlight- ing to that element (be it a link or a form control). This is known as focus. From what you have learned already about XHTML, you know that not every element in the document receives this focus. In fact, it is only the elements that a user can interact with, such as links and form con- trols, that can receive focus. Indeed, if a user is expected to interact with an element, that element must be able to receive focus. An element can gain focus in three ways: ❑ An element can be selected using a pointing device such as a mouse or trackball. ❑ The elements can be navigated between using the keyboard — often using the Tab key (or Shift+Tab to move backward through elements). The elements in some documents can be given a fixed tabbing order, indicating the order in which elements gain focus. ❑ You can use a system such as a keyboard shortcut known as an access key to select a particular ele- ment. For example, on a PC you would likely press the Alt key plus an access key (such as Alt+E), whereas on a Mac you would press the Control key with an access key (such as Control+E). Tabbing Order If you want to control the order in which elements can gain focus, you can use the tabindex attribute to give that element a number between 0 and 32767, which forms part of the tabbing order. Every time the user presses the Tab key, the focus moves to the element with the next highest tabbing order (and again, Shift+Tab moves focus in reverse order). The following elements can carry a tabindex attribute: The tabindex attribute was first supported in Netscape 6 and IE 4, but older browsers just ignore this attribute, so it is safe to use it in all documents. After a user has tabbed through all elements in a document that can gain focus, then focus may be given to browser features (most commonly the address bar). 173
  4. Chapter 5: Forms To demonstrate how tabbing order works, the following example gives focus to the checkboxes in a different order than you might expect (ch05_eg18.html): One Two Three Four Five Six Seven Eight Nine Ten In this example, the checkboxes receive focus in the following order: 4, 8, 1, 3, 10, 6, 2, 9, 5, 7 Figure 5-22 shows how Firefox 2 for PC will, by default, give a yellow outline to form elements as they gain focus (other browsers give different outlines — Internet Explorer uses blue lines). I have zoomed in on the item in focus so you can see it in closer detail. Figure 5-22 You should always start your tabindex values with 1 or higher, rather than 0, because elements that could gain focus but do not have a tabindex attribute are given a value of 0 and are navigated in the order in which they appear after those with a tabindex have been cycled through. If two elements have the same value for a tabindex attribute, they will be navigated in the order in which they appear in the document. Note that if an element is disabled, it cannot gain focus and does not participate in the tabbing order. 174
  5. Chapter 5: Forms Access Keys Access keys act just like keyboard shortcuts. The access key is a single character from the document’s character set that is expected to appear on the user’s keyboard. When this key is used in conjunction with another key (such as Alt on Windows and Control on an Apple), the browser automatically goes to that section (exactly which key must be used in conjunction with the access key depends upon the operating system and browser). The access key is defined using the accesskey attribute. The value of this attribute is the character (and key on the keyboard) you want the user to be able to press (in conjunction with the other key that is dependent upon the operating system and browser). The following elements can carry an access key attribute: The accesskey attribute was first supported in Netscape 6 and IE 4, but older browsers just ignore these attributes, so it is safe to use them in all documents. To see how access keys work, you can revisit the example of a competition form (ch05_eg17.html), which was covered in the section “Structuring Your Forms with and Elements” earlier in this chapter. Now the accesskey attributes can be added to the elements: Contact Information (ALT + C) Competition Question Tiebreaker Question (ALT + T) Enter competition The new version of this file is ch05_eg19.html in the download code. (Extra elements have been added to show how the screen scrolls to the appropriate section when an access key is used.) As a hint to users that they can use the access keys as shortcuts, information has also been added to the information in the element in two ways: ❑ In brackets after the title ❑ By underlining the access key itself Figure 5-23 shows how this updated example looks in a browser. The effect of an access key being used depends upon the element that it is used with. With elements, such as those shown previously, the browser scrolls to that part of the page automatically and gives focus to the first form control in the section. When used with form controls, those elements gain focus. As soon as the element gains focus, the user should be able to interact with it (either by typing in text controls or pressing the Enter or Return key with other form controls). When using letters a–z, it does not matter whether you specify an uppercase or lowercase access key, although strictly speaking it should be lowercase. 175
  6. Chapter 5: Forms Figure 5-23 Disabled and Read-Only Controls Throughout the chapter you have seen that several of the elements can carry attributes called disabled and readonly: ❑ The readonly attribute prevents users from changing the value of the form control themselves, although it may be modified by a script. A name/value pair of a readonly control will be sent to the server. Its value should be readonly. ❑ The disabled attribute disables the form control so that users cannot alter it. A script can be used to re-enable the control, but unless a control is re-enabled, the name/value pair will not be sent to the server. Its value should be disabled. The readonly and disabled attributes were implemented in Netscape 6 and IE 5, although older browsers ignore them, so you can add these attributes to all documents. You should be aware, however, that because older browsers ignore these attributes, users with older browsers would still be able to interact with form controls that have readonly or disabled attributes. 176
  7. Chapter 5: Forms A readonly control is particularly helpful when you want to stop visitors from changing a part of the form, perhaps because it cannot change (such as in the case of terms and conditions) or because you want to indicate to a user something that they have already said, or when. You often see readonly controls for user agreements and in the body of e-mail forms that allow you to e-mail a web page to a friend. The disabled attribute is particularly helpful when preventing users from interacting with a control until they have done something else. For example, you might use a script to disable a submit button until all of the form fields contain a value. The following table indicates which form controls work with the readonly and disabled attributes. Element readonly disabled Yes Yes Yes Yes No Yes No Yes No Yes No Yes No Yes No Yes No Yes No Yes The following table indicates the main differences between the readonly and disabled attributes. Attribute readonly disabled Can be modified Yes by script, not by user Not while disabled Will be sent to server Yes Not while disabled Will receive focus Yes No Included in tabbing order Yes No 177
  8. Chapter 5: Forms Sending Form Data to the Ser ver You’ve already learned about the submit button, which the user presses to initiate the sending of form data to the server, but this book has not yet covered the difference between the HTTP get and HTTP post meth- ods. You might remember that you can specify which of these two methods is used by adding the method attribute to the element — just as all of the examples in this chapter have done. The method attribute can take one of two values, either get or post, corresponding to the HTTP methods used to send the form data. If the element does not carry a method attribute, then by default the get method will be used. If you are using a file upload form control, you must choose the post method (and you must set the enctype attribute to have a value of multipart/form-data). HTTP get When you send form data to the server using the HTTP get method, the form data is appended to the URL specified in the action attribute of the element. The form data is separated from the URL using a question mark. Following the question mark you get the name/value pairs for each form control. Each name/value pair is separated by an ampersand (&). For example, take the following login form, which you saw when the password form control was introduced: Username: Password: When you click the submit button, your username and password are appended to the URL http://www .example.com/login.aspx like so in what is known as the query string: http://www.example.com/login.aspx?txtUsername=Bob&pwdPassword=LetMeIn Note that, when a browser requests a URL with any spaces or unsafe characters (such as /, \ , =, &, and +, which have special meanings in URL), they are replaced with a hex code to represent that character. This is done automatically by the browser, and is known as URL encoding. When the data reaches the server, the server will usually un-encode the special characters automatically. One of the great advantages of passing form data in a URL is that it can be bookmarked. If you look at searches performed on major search engines such as Google, they tend to use the get method so that the page can be bookmarked. The get method, however, has some disadvantages. Indeed, when sending sensitive data such as the password shown here, or credit card details, you should not use the get method because the sensitive data becomes part of the URL and is in full view to everyone (and could be bookmarked). 178
  9. Chapter 5: Forms You should not use the HTTP get method when: ❑ You are updating a data source such as a database or spreadsheet (because someone could make up URLs that would alter your data source). ❑ You are dealing with sensitive information, such as passwords or credit card details (because the sensitive form data would be visible as part of a URL). ❑ You have large amounts of data (because older browsers do not allow URLs to exceed more than 1,024 characters — although the recent versions of the main browsers do not have limits). ❑ Your form contains a file upload control (because uploaded files cannot be passed in the URL). ❑ Your users might enter non-ASCII characters such as Hebrew or Cyrillic characters. In these circumstances, you should use the HTTP post method. HTTP post When you send data from a form to the server using the HTTP post method, the form data is sent trans- parently in what is known as the HTTP headers. While you do not see these headers, they are sent in clear text and cannot be relied upon to be secure (unless you are sending data under a Secure Sockets Layer, or SSL). If the login form you just saw was sent using the post method, it could look something like this in the HTTP headers: User-agent: MSIE 5.5 Content-Type: application/x-www-form-urlencoded Content-length: 35 ...other headers go here... txtUserName=Bob&pwdPassword=LetMeIn Note that the last line is the form data, and that it is in exactly the same format as the data after the ques- tion mark in the get method — it would also be URL — encoded if it contained spaces or any characters reserved for use in URLs. There is nothing to stop you using the post method to send form data to a page that also contains a query string. For example, you might have one page to handle users that want to subscribe to or unsubscribe from a newsletter, and you might choose to indicate whether a user wanted to subscribe or unsubscribe in the query string. Meanwhile, you might want to send their actual contact details in a form that uses the post method because you are updating a data source. In this case, you could use the following element: The only issue with using the HTTP post method is that the information the user entered on the form cannot be bookmarked in the same way it can when it is contained in the URL. So you cannot use it to retrieve a page that was generated using specific form data as you can when you bookmark a page gen- erated by most search engines, but it is good for security reasons. 179
  10. Chapter 5: Forms Try It Out The Registration Form Revisited It is time to revisit the registration form from the earlier Try It Out section in this chapter. This time you add some more fields into it, and make it more usable. 1. Open the file registration.html that you made earlier in the chapter and save it as registration2.html so that you have a different copy to work with. 2. You should create elements for all of the form controls. This involves putting the instructions for that control inside a element. This element should carry the for attrib- ute, whose value is the value of the id attribute on the corresponding form control, like this one: User name: 3. You have to label the two radio buttons individually: Gender: Male Female If you remember the last chapter’s discussion of table linearization for screen readers, then this should work fine for most users. If, however, another column were to the right with unrelated information (such as ads) this could confuse readers, so the table for the form controls should hold only the controls and their labels. 4. Next you are going to add four new text boxes after the username and password. The first text input will be to confirm the password and then there will be an empty row. This will be followed by two text inputs: one for the user’s first name and one for the user’s last name. Then there will be another empty row, followed by an input for the user’s e-mail address: Confirm Password:    180
  11. Chapter 5: Forms First name: Last name:    Email address: 5. Now it is time to split the form into two sections using the element. The first section will indicate that it is for information about the user (containing username, password, name, e-mail, and gender details). The second section is for information about the company (how the user found the site and if they want to be on the mailing list). Both elements will carry access keys. Here is the element for the second section of the form: About Us (ALT + U) How did you hear about us?: Select answer Another website Magazine ad From a friend Other    Please select this box if you wish to be added to our mailing list We will not pass on your details to any third party. This extended registration form is now a lot more usable. If you save the file again and open it in your browser, you should find something that resembles Figure 5-24. 181
  12. Chapter 5: Forms Figure 5-24 How It Works You should be familiar with most of what is going on here, but let’s just address a few key points. ❑ The form has been divided up into sections using the element. This added structure makes it easier to use the form, as the user knows what section he or she is in. ❑ The accesskey attributes, which provide keyboard shortcuts, are particularly helpful if you are creating long forms, so that users can immediately go to the relevant section. In reality, the accesskey attributes are more likely to be of use when creating a site that people will use fre- quently, rather than a form that users will use only a few times. Users tend to use the shortcuts only if they are familiar with the form already and want to skip between the sections. ❑ As you will see in Chapter 10, if you are creating a particularly long form, it may be a good idea to split the form up into several pages. ❑ The elements are of particular help to those who use screen readers. It ensures that the users know what they are supposed to be entering into which form control. ❑ When splitting up your page using the element, make sure that your elements nest correctly. You cannot just place elements between rows of a table. 182
  13. Chapter 5: Forms Summar y This chapter has introduced you to the world of creating online forms, which are a vital part of many sites. In most cases when you want or need to directly collect information from a visitor to your site you will use a form, and you have seen several different examples of forms in this chapter. From simple search boxes and login pages to complex online order forms and registration processes, forms are a vital part of web design. You have learned how a form lives inside a element and that inside a form there are one or more form controls. You have seen how the element can be used to create several kinds of form con- trols, namely single-line text input controls, checkboxes, radio buttons, file upload boxes, buttons, and hidden form controls. There are also the elements for creating multiple line text inputs and the and elements for creating select boxes. Once you have created a form with its form controls, you need to ensure that each element is labeled properly so that users know what information they should enter or which selection they will be making. You can also organize larger forms using the and elements and aid navigation with tabindex and accesskey attributes. Finally, you learned when you should use the HTTP get or post methods to send form data to the server. Next, it is time to look at the last of our core XHTML chapters, which covers framesets. You will see more about form design in Chapter 12, which covers some design issues that will make your forms easier to understand. Exercises The answers to all of the exercises are in Appendix A. 1. Create an e-mail feedback form that looks like the one shown in Figure 5-25. Figure 5-25 183
  14. Chapter 5: Forms Note that the first text box is a readonly text box so that the user cannot alter the name of the person the mail is being sent to. 2. Create a voting or ranking form that looks like the one shown in Figure 5-26. Figure 5-26 Note that the following element was added to the of the document to make each column of the table the same fixed width, with text aligned in the center (you’ll see more about this in Chapter 7). Voting td {width:100; text-align:center;} 184
  15. 6 Frames Frames divide a browser window into several separate pieces or panes, each pane containing a separate XHTML page. One of the key advantages that frames offer is that you can then load and reload single panes without having to reload the entire contents of the browser window. A collection of frames in the browser window is known as a frameset. The window is divided into frames much as tables are organized: into rows and columns (although they are usually relatively basic in structure). The simplest of framesets might just divide the screen into two rows, whereas a complex frameset could use several rows and columns. In this chapter you learn the following: ❑ How to create a frameset document with multiple frames ❑ How to create inline frames (or iframes), which are single windows that sit within another page ❑ How to deal with users whose browsers cannot use frames I should warn you early on that there are actually very few cases in which I would consider using frames, although this is a matter of preference and I explain my reasons why in the second section of this chapter — after a simple example that helps you understand what frames really are. Introducing the Frameset To help you understand frames, Figure 6-1 shows you a frameset document in a browser. This frameset divides the page into three parts, and each separate part of the page is a separate XHTML document.
  16. Chapter 6: Frames Figure 6-1 You may remember from Chapter 1 that, when writing a frameset document, you use a different DOCTYPE declaration. This is because frameset documents use a few elements in different ways than other XHTML documents. To create a frameset document, first you need the element, which is used instead of the element. The frameset defines the rows and columns your page is divided into. Each frame is then repre- sented by a element. You also need to learn the element, which provides a message for users whose browsers do not support frames. To get a better idea of how frames work, here is the code for the frameset shown previously in Figure 6-1 (ch06_eg01.html): Frames example 186
  17. Chapter 6: Frames This site uses a technology called frames. Unfortunately, your browser does not support this technology. Please Please upgrade your browser and visit us again! In practical terms, the new DOCTYPE declaration does little more than allow you to use these frame- related elements. You already know that there is no element as it has been replaced with the element; also, there should be no markup between the closing tag and the opening tag, other than a comment if you want to include one. As you will see shortly, the element must carry the two attributes rows and cols, which specify the number of rows and columns that make up the frameset. In our example there are just three rows, the first being 150 pixels high, the third just 100 pixels high, and the second taking up the rest of the page (the asterisk is used to indicate that the remaining part of the page should be used in this place). Inside the element are the empty elements. The elements indicate a URL of the document that will be loaded into that frame; the URL is specified using the src attribute (this is very similar to the way an image file is specified in an element). There is also a element whose contents will be displayed if the user’s browser does not support frames. Three separate documents are displayed in the one browser window in this example: ❑ top_frame.html ❑ main_frame.html ❑ bottom_frame.html You should be able to see which part of the window each of these pages corresponds to fairly easily in Figure 6-1, shown previously. To see you another idea of how frames can work, look at Figure 6-2, which shows a page that uses horizontal and vertical frames. (This shows the similarity between the way in which simple tables are sometimes used to divide up pages and how frames can be used.) Even though Netscape has supported frames since version 2, and IE introduced them in version 3, frames didn’t make it into HTML until version 4.0. Now that you have a good idea of how a frameset document appears, before you take a closer look at the syntax, let’s just have a look at when you might want to use frames. 187
  18. Chapter 6: Frames Figure 6-2 When to Use Frames It is quite rare to see frames in use these days. Personally, there are very few circumstances in which I would suggest that you use frames in a page. The cases in which I think frames are useful include: ❑ When you want to display a lot of content in one single page and you cannot split the document into separate pages, then a frame might be of use to create a navigation bar that links to the sub- sections of the long document. ❑ When you have a lot of data in one part of the page that you do not want the user to have to reload while another part of the page changes. Examples might include a photography site where you have lots of thumbnails in one frame, and the main picture in another. Rather than reloading the thumb- nails each time if a visitor wants to look at a new main picture, you can just reload the main picture. As you have seen in the first example, for each frame you have in a layout, you need to have a file that acts as its content (each frame is essentially its own web page), so the number of files in your site quickly grows. You therefore need to be particularly careful with your file structure so that you do not get lost in a sea of extra files. A couple of other drawbacks you should be aware of with frames are as follows: ❑ Search engines often link to the content of individual frames rather than the frameset (or group of frames) that the user sees (and you have to use JavaScript in each frame to reload the frameset if a visitor lands on an individual frame). ❑ Some browsers do not print well from framesets (and will just print one frame at a time). ❑ The browser’s Back button might not work as the user expects. 188
  19. Chapter 6: Frames ❑ Some smaller devices cannot cope with frames, often because their screen is not big enough to be divided up. ❑ It can be hard to achieve a nice layout because users with a lower resolution monitor than the designer can end up seeing only a portion of what you intend them to see, while users with a higher resolution monitor than the designer may end up with large gaps around the edges of the frames. ❑ If you have a navigation frame loading different pages into a “main frame,” it is hard to create a navigation bar that tells users which page they are on (because the other frame loads the new page without telling the navigation bar). I should also mention that when a web developer wants to create a page where only part of the page is refreshed (rather than the whole page), it is becoming increasingly common to use a technique referred to as AJAX (Asynchronous JavaScript and XML). While you know my opinion on frames, if you think their advantages outweigh the disadvantages, then you should use them. So let’s take a look at the syntax for using frames in a little more detail. The Element The element replaces the element in frameset documents. It is the attributes of the element that specify how the browser window will be divided up into rows and columns. These attributes are as follows: ❑ cols specifies how many columns are in the frameset. ❑ rows specifies how many rows are in the frameset. The element contains a element for each frame of the document (or each cell of the grid constructed by the element) and a element to indicate what should be displayed if the user’s browser does not load frames. In addition to the rows and cols attributes, the frameset element can also take the following attributes: class id onload onunload rows style title Most browsers also support the following well-used attributes (some of which are covered here because of their popularity). They are not, however, part of the W3C recommendation. onblur onfocus border bordercolor frameborder framespacing The cols Attribute The cols attribute specifies how many columns are contained in the frameset and the size of each column. You have to provide a value to indicate the width for each of the columns in your frameset, and the number of values you provide (each separated by a comma) indicates how many columns there are in the document. For example, here there are three columns: the first takes up 20 percent of the width of the browser window, the second takes up 60 percent, and the third takes the last 20 percent: cols=”20%, 60%, 20%” 189
  20. Chapter 6: Frames Because there are three values, the browser knows that there should be three columns. You can specify the width of each column in one of four ways: ❑ Absolute values in pixels ❑ A percentage of the browser window (or parent frame if you have one frameset sit inside another — which is known as a nested frame) ❑ Using a wildcard symbol ❑ As relative widths of the browser window (or parent frame) You can mix and match these different ways of specifying column widths, but note the precedence they take (discussed after the four methods). If you do not specify a cols attribute, then the default value is 100 percent, so if you do not specify a cols attribute, then there will be one column that takes up 100 percent of the width of the browser. Absolute Values in Pixels To specify the width of a column in pixels, you just use a number. (You do not need to use px or any other characters after the number.) For example, here are three columns: the first is 100 pixels, the second is 500 pixels, and the third takes up the remainder of the page (using the wildcard symbol *). cols=”100, 500, *” If you use absolute values only, and the width of the window is less or more than the specified values, then the browser will adjust the width of each column in proportion to the width of the browser window. So, if you want three columns of 100 pixels, you might specify it like this: cols=”100, 100, 100” However, if the browser window were 600 pixels wide, you would end up with three columns of 200 pixels. Therefore if you really want to specify fixed absolute widths that won’t grow, use a wildcard character after the third column and either make the content of the fourth frame blank or do not include a element for it: cols=”100, 100, 100, *” Interestingly, if you have four columns 200 pixels wide, and the browser window is only 600 pixels wide, your columns would all be squashed proportionately to 150 pixels wide; the window will not use scroll- bars to make the page 800 pixels wide. A Percentage of the Browser Window or Parent Frame To specify the width of a column as a percentage of a window (or, if you use nested frames, which you will meet later in the chapter, a percentage of the parent frame) you use a number followed by the percent sign. For example, the following attribute value specifies two columns, one of 40 percent and another of 60 per- cent of the browser window: cols=”40%, 60%” 190
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