An Introduction to DHTML
Another acronym in the alphabet soup of the WWW(!), DHTML stands for Dynamic Hyper Text Markup Language. This is not one technology like JavaScript or HTML but a combination of the following 4:
- HTML
- CSS
- JavaScript
- DOM
Lets look at each technology in particular and find out how they are tied together to become DHTML.
HTML:
The de-factor Markup Language of the internet, this is used to structure and create a web – page with content. The first generation of HTML was geared towards just displaying information to the user. Later, user inputs were accepted and processed. User inputs are given using html controls like input boxes, radio buttons, check boxes, etc. But this still had a distinct drawback – once the page was sent from the server to the client, the user would only see what was served the first time and any changes in the page needed another round trip back to the server and waiting till the page loaded. Thus HTML wasn’t completely dynamic.
CSS:
Think of having a web-page with a common layout for each page and certain properties like Font Size, Background, etc. being consistent across the site. This is one of the primary rules of good User Interface and Web Page design viz., to give the user a consistent experience across the web site. Earlier, if the Font Size needed to be changed for a few controls in a HTML page, the setting was done at the individual control level. CSS allows one to create classes with particular properties and use them across an entire website.
As an example, consider a corporate website with some disclaimers and copyright notices to be displayed in a smaller font at the bottom of each page. For this, a class called NormalFontSize with a size of 10 and a SmallFontSize with a size of 14 can be created. So all the controls needing to use the smaller font would have their class property set to SmallFontSize and the remaining can be set to NormalFontSize. This is a very trivial example but the use of CSS can be extended to all acceptable properties for any HTML control. If you need to change your font size from Times New Roman to Arial, all one has to do is create a new class for Arial and point all the controls to this class, which is much simpler and has less scope for error compared to accessing and changing individual controls across each page of a website.
JavaScript:
The true dynamism of Web comes into fore only with Scripting Languages and JavaScript is the most popular one used. It is generally used to handle user input at the client side. For example, when the user is typing a form and enters 1234 in the name field, a simple check can be made at the LostFocus event of the Input TextBox instead of the content going all the way to the server for the user to know that 1234 is invalid in the Name box.
When one wants to add dynamism to a web page, the principal tool involved would be JavaScript which would add content based on the user actions.
DOM:
In the simplest terms, Document Object Model is the way in which all the elements in a web page including the HTML controls are referenced or tied together and identified w.r.to the page. This envisions the current Document or Web Page as a tree and all the controls as nodes of the tree. Any element in the tree can thus be accessed by traversing through it. GetElementById is a common way to identify and act on a particular element.
DHTML:
So how do all these technologies mesh to become DHTML? Let’s take a page where based on the country of origin given as user input via radio buttons, we need to alter the page dynamically and put in a different font color for a textbox control. In the normal scenario involving a server, one approach is to have 3 different pages and once the user input in page 1 is sent back to the server, the server sends back either page 2 or page 3 with the appropriate color for each country.
The alternate approach - do the following operation on the OnClick() event of the radio button in JavaScript:
a) Traverse and reach the particular HTML textbox using DOM.
b) If its country 1, add CSS class 1 to the textbox control and add CSS class 2 otherwise.
So as soon as the user chooses his country of origin, he will be able to type in a particular color in the textbox next to it.
Sites incorporating DHTML have exploded over the past few years. Earlier, this was used to do actions like adding flying text and making parts of the page blink, in short, lots of gimmicks which had no value addition to the user. These have been replaced and meaningful content and design are being served by taking advantage of this technology.
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