A World Wide Web client or browser connects to a World Wide Web server and asks for a document. It knows which server to connect to and which document to ask for from the information given in a URL (see chapters 3 and 5).
When the server replies with the contents of the document, the client must display it to the user.
Now, a document can contain plain text to be displayed, it can contain HTML text which needs special formatting, or it can contain multimedia data such as a picture or an audio clip to be displayed or played. The client must decide what type of data the document contains, and so what to do with it. The information about what type of data the document contains is conveyed in a few additional lines of information the WWW server adds to the top of the document. In particular, the line that says what type of data is in the document is called the content type (see sections and for more details).
Some content types, such as plain text or HTML text, will be dealt with by the client itself. Plain text is easy to display. HTML takes a little more intelligence, as it needs laying out neatly on the screen in appropriate fonts and the paragraphs of text need to be wrapped at appropriate places if the screen is too narrow to display them. HTML can also contain embedded images (see section ). To display an embedded image the browser will need to make another connection to a server to retrieve the image data, and then it'll need to leave enough space in the text to insert the picture. See figure 4.1 for an example.
Figure 4.1: EIT MacWeb displaying images on an Apple Macintosh
Some links in an HTML document will also result in documents being retrieved that contain data that cannot be displayed by the browser itself. In most cases the browser will then send the document data to a separate viewer program to display the image or play the video or audio clip that the document data represents. See section 4.6 for how a browser knows which viewer to start up.