Images currently enjoy a special status among the various media that can be included within an HTML or XHTML document and displayed inline with other content by all but a few browsers. Sometimes, however, as we discussed earlier in this chapter, you may also reference images externally, particularly large ones in which details are important, but not immediately necessary to the document content. Other multimedia elements, including digital audio and video, can be referenced as separate documents external to the current one.

You normally use the anchor tag (<a>) to link external multimedia elements to the current document. Just like other link elements selected by the user, the browser downloads the multimedia object and presents it to the user, possibly with the assistance of an external application or plug-in. Referenced content is always a two-step process: present the document that links to the desired multimedia object, then present the object if the user selects the link. Section 6.3.1, "The <a> Tag"

In the case of images, you can choose how to present images to the user: inline and immediately available via the <img> tag, or referenced and subsequently available via the <a> tag. If your images are small and critical to the current document, you should provide them inline. If they are large or are only a secondary element of the current document, make them available as referenced content via the <a> tag.

If you choose to provide images via the <a> tag, it is sometimes a courtesy to your readers to indicate the size of the referenced image in the referencing document and perhaps provide a thumbnail sketch. Users can then determine whether it is worth their time and expense to retrieve it.

You reference any external document, regardless of type or format document via a conventional anchor (<a>) link:

Browsers identify and specially handle multimedia files from one of two different hints: either from the file's Multipurpose Internet Mail Extension (MIME) type provided by the server or from a special suffix in the file's name. The browser prefers MIME because of its richer description of the file and its contents, but will infer the file's contents (type and format) of the object by the file suffix: .gif or .jpg, for GIF and JPEG encoded images, for example, or .au for a special sound file.

Since not all browsers look for a MIME type, nor will they all be correctly configured with helper applications by their users, you should always use the correct file suffix in the names of multimedia objects. See Table 5-1 earlier in this chapter.

Creating effective links to external multimedia documents is critical. The user needs some indication of what the object is and perhaps the kind of application the linked object needs to execute. Moreover, most multimedia objects are quite large, so common courtesy tells us to provide users with some indication of the time and expense involved in downloading it.

The Web can deliver nearly any type of electronic document, not just graphics, sound, and video files. To display them, however, the client browser needs a helper application installed and referenced. Recent browsers also support plug-in accessory software and, as described in Chapter 12, "Executable Content", may extend the browser for some special function, including inline display of multimedia objects.

For example, consider a company whose extensive product documentation was prepared and stored in some popular layout application such as FrameMaker, Quark XPress, or PageMaker. The Web offers an excellent way for distributing that documentation over a worldwide network, but converting to HTML or XHTML would be too costly at this time.

The solution is to prepare a few HTML or XHTML documents that catalog and link the alternative files and invoke the appropriate display applet. Or, make sure that the users' browsers have the plug-in software or are configured to invoke the appropriate helper application. Adobe's Acrobat Reader is a very popular plugin, for example. If the document is in Acrobat (.pdf ) format, then if a link to an Acrobat document is chosen, the tool is started and accordingly displays the document, often right in the browser's window.