Rules, Images, and Multimedia - HTML and XHTML: The Definitive Guide - page 227

The lowsrc attribute's value, like src , is the URL of an image file. Nets-
cape before version 6 would load and display the lowsrc image when
it first encountered the <img> tag. Then, when the document had been
completely loaded and the user could read it, Netscape would retrieve
the image specified by the src attribute.
No other browser besides Netscape versions 4 and earlier supports
lowsrc . Netscape version 6 simply uses the dimensions of the lowsrc im-
age to temporarily allocate display space for the image as it renders
the document. The earlier versions of Netscape also used the lowsrc di-
mensions to resize the final image, which you could exploit for some
special effects. This no longer works. Instead, we recommend that you
eschew the Netscape extension and explicitly allocate image space with
the height and width attributes described later in this chapter.
5.2.6.3. The alt and longdesc attributes
The alt attribute specifies alternative text the browser may show if im-
age display is not possible or is disabled by the user. Especially favored
by visually impaired users, the popular browsers also let us choose to
display alt text along with the image. So although it's an option, it's one
we highly recommend you exercise for most images in your document.
This way, if the image is not available, the user still has some indication
of what's missing. And for users with certain disabilities, alt often is the
only way they can appreciate your images.
In addition, Internet Explorer displays the alternative description in a
text box when users pass the mouse over the image. Accordingly, you
might embed short, parenthetical information that pops up when users
pass over a small, inline icon, such as that shown in Figure 5-9 .