Okay, you’ve written about Cascading Style Sheets (CSS) and eXtensible Mark-up Language (XML), what about the other new web tool, Dynamic HTML (DHTML)?  Where does it fit in with all of these techniques to format Web pages?  

—Determined to have DHTML Defined
 

Dear DDD—

You are correct.  Dynamic HTML is one more way to liven up web pages.  At the most basic level, DHTML will provide two things:  1) web pages that can change their content and the way it is displayed “on the fly”; and 2) mechanisms that allow this activity to take place on the client (your web browser on your personal computer) rather than on the server (the computer located elsewhere where the documents are stored).  Mechanisms currently exist (embedded Java applets and ActiveX) that appear to change the content of web pages on the fly, but these mechanisms require that the browser go back to the server and retrieve a new copy of a page.  With DHTML, everything the browser needs to make changes in the page is transferred to the client the first time it retrieves the page.  Two benefits of this technology is the speed at which new information can be displayed to the user and the reduced overhead on the network and the server.

What does it mean to create new web pages on the fly?  Let’s say you have a web page with a series of buttons on it.  Associated with each button you have a different menu of activities from which the user can choose.  Without DHTML, the only way the user can see those activities is to click on the button, and a new page is retrieved revealing the menu associated with that button.  With DHTML, the original web page is designed in such a way that when the mouse pointer happens to be positioned above a button, a menu automatically appears below the button, without the browser having to retrieve a new copy of the page.  As the mouse moves over other buttons, one menu disappears and the next one appears.  Only when the user finally selects an item from the menu will a new web page be retrieved from the server.  In this example, DHTML allows the user to see, very quickly, all of the different menu choices and more easily decide where to go next.  Since space is a premium on web pages, the ability to hide and show information as it is needed by the user is particularly valuable.

Another benefit of DHTML is the display of data in a table or from a database.  With DHTML, an author may specify the number of rows to display, and additional rows may be “hidden” on the page for easy display when the user needs them.  All of the information retrieved from a database is re-trieved in one trip to the server.  The user then decides how much of the information to view.  Additionally, any sorting of the data can take place on the client’s machine, not on the server —once again decreasing the amount of time it takes to display the information and the overhead on the network and the server.

Stanek details 5 features of DHTML:

1) Dynamic Content and Style Changes allow the user to change appearance of a Web page after it has been loaded, as described above.

2) Element Positioning and Animation make use of X, Y, and Z coordinates to allow an “object” to be placed on a page very  precisely.  Specifically the Z coordinate allows you to create “layers” of information.  With Netscape, the layering of information is achieved using the non-standard <layer> tag.

3) Multimedia Effects can be created without writing complicated scripts, using plug-ins, or having the heavy overload associated with large animated image files.

4) Dynamic Fonts eliminate the fear of the client computer not having the necessary font to view a page as the designer wants it displayed.  The source for the appropriate font is included in the web page, and it is made temporarily available for the client’s computer to use while viewing the pages that require the font.

5) Data Binding ties elements in the web page (like table cells or a specific paragraph) to a data source.  When the page is loaded, the current information from the data source is read and dynamically inserted.

Could these features of DHMTL be too good to be true?  Possibly. There is one fairly significant problem with DHTML — currently, no standards have been approved by the WWW Consortium.  Because there are no standards, both Internet Explorer 4.0 and Netscape 4.0 have implemented DHTML, but they use very different methods of implementation.  Microsoft has extended existing standards, such as Cascading Style Sheets Standards; whereas Netscape has chosen to focus on new ways of doing things, most noticeably with its development of the <layer> tag. Although, Microsoft appears to have no intention of adopting any of the Netscape DHTML features, such as the <layer> tag; Netscape currently sup-ports some of the Microsoft DHTML features.  Meanwhile, web designers are left in a quandary as to whether they should develop DHTML pages for Internet Explorer or for Netscape or try to develop something viewable by either browser or just not to develop in DHTML at all. 

Another caveat about DHTML is that, although it is much easier for the non-programmer to learn than the Java programming language or ActiveX, it still has a steeper learning curve than the one associated with straight HTML, or even Cascading Style Sheets or XML.  It doesn’t use the Java programming language, but it does use scripts, and the more comfortable someone is with programming languages, the easier it will be to implement DHTML. 

If you are really interested in using DHTML to make library web pages or instruction web pages more effective, look at examples of what DHTML can do with both Internet Explorer 4.0 and Netscape 4.0.  Read more about the differences between the two browsers’ implementations of DHTML.  Stay attuned to the deliberations of the WWW Consortium as they develop the DHTML standards.  Look into Macromedia’s DreamWeaver software which is an advanced web editor that includes the capability to develop DHTML web pages.  Make use of the tutorials and  examples provided by Microsoft, Netscape, and others to create some DHTML web pages that will work on both Internet Explorer and Netscape, but remember that in presenting any information via DHTML, you automatically exclude the users of pre-4.0 versions of Internet Explorer or Netscape.  Be sure to check how the DHTML implementation displays in older web browsers.

For more information:

Campbell, Bruce and Rick Darnell.  Teach Yourself Dynamic HTML in a Week.  Indianapolis, IN:Sams.net, 1997.

Dobson, Rick.  “Dynamic HTML Explained, Part I.” Byte (November 1997): 53-54.  (Also available online: http://www.byte.com/art/9711/sec4/art5.htm.)

Dobson, Rick.  “Dynamic HTML Explained, Part II.” Byte (December 1997): 61-62.  (Also available online: http://www.byte.com/art/9712/sec4/art6.htm.)                                  
Dobson, Rich.  “Dynamic HTML Explained, Part III.”  Byte (January 1998): 43-44.  (Also available online: http://www.byte.com/art/9801/sec4/art1.htm.) 

“Dynamic HTML in Netscape Communicator.” <URL: http://developer.netscape.com/docs/manuals/communicator/dynhtml/>

“Dynamic HTML Zone Resources:  Tutorials.” <URL: http://www.dhtmlzone.com/resources/tutorials.html>

“Dynamic HTML Index.”  <URL: http://www.all-links.com/dynamic/learn.html>

Goldberg, Harold.  “Power for Expert’s Hands (Macromedia’s DreamWeaver Web Authoring Software).”  New York Times (May 28, 1998): D8.

Lynch, Kevin.  “Cross-Browser Dynamic HTML.”  <URL: http://www.dhtmlzone.com/articles/dhtml.html>

“Microsoft DHTML, HTML, and CSS.”  <URL: http://www.microsoft.com/workshop/author/default.asp>

“Microsoft’s Samples, Headers, and Libraries.” <URL: http://www.microsoft.com/gallery/samples/default.asp>

Needleman, Rafe.  “Dynamic HTML:  A Web Revolution.”  <URL: http://www.cnet.com/Content/Voices/Needleman/031097/index.html>

Needleman, Rafe.  “What’s the Catch?” <URL:  http://www.cnet.com/Content/Voices/Needleman/031097/index2.html> 

“Netscape Communicator Demonstrations.” <URL: http://form.netscape.com/comprod/products/communicator/ beta_features.html>

Randall, Neil.  “Dynamic HTML:  More Client, Less Server.”  PC Magazine (July 1997):246-248.  (Also available online: http://www.zdnet.com/pcmag/issues/1613/pcmg0066.htm.)

Stanek, William R.  “Creativity and Control.”  PC Magazine (January 20, 1998): 233-237.

Wong, Charmaine.  “What is So Dynamic About Dynamic HTML?”  <URL: http://www.rcc.ryerson.ca/schools/rta/brd038/papers/1997/dynahtm1.htm>