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What is a Form in XHTML, and what is the role of CGI in a Form?

The XHTML form itself is the interface for the user to enter data, but all of the actual data processing is performed on the server using applications that usually reside in the Common Gateway Interface (CGI).

The Common Gateway Interface is the communications bridge between the Internet and the server. Using programs called “scripts,” CGI can collect data sent by a user via the Hypertext Transfer Protocol (HTTP) and transfer it to a variety of data processing programs, including spreadsheets, databases, or other software running on the server. The data processing software then can work with the data and send a response back to CGI, and then on to the user.

Explain what interlacing and progressive display is, the advantages and the disadvantages of such methods

Most Web-capable graphics editors let you save images in an interlaced (progressive) format. Interlacing and progressive formats generally are the same thing—the gradual display of a graphic in a series of passes as the data arrives in the browser. Each additional pass of data creates a clearer view of the image until the complete image is displayed.

The only real advantage to displaying graphics in the interlaced or progressive method is that users immediately see at least a blurred view of the complete image, giving them something to look at while waiting for the entire graphic to download.

The disadvantage of choosing this display method is that older browsers may not display the graphic properly, and more processing power is needed on the user’s machine to render the image. The use of these methods has declined as increased connection speeds become available.