HTML 4.0 Sourcebook
(Publisher: John Wiley & Sons, Inc.)
Author(s): Ian S. Graham
ISBN: 0471257249
Publication Date: 04/01/98

where the hash character indicates that the remaining text is a fragment identifier. Selecting “Poisonous non-mushrooms” links the user back to the place in the document marked by the <A NAME="poison">...</A> anchor.

This location can be accessed from another document via an anchor of the form:

<A
HREF="http://www.site.edu/slimy/toads.html#poison">Poisonous</A>

assuming http://www.site.edu/slimy/toads.html to be the URL of the document in question. Clicking on the word “Poisonous” then links the user to the document toads.html and to the location marked by the <A NAME="poison">...</A> anchor. This need not be a full URL, but can also be a partial URL should the two documents lie on the same server. These issues are discussed in more detail in Chapter 2 (Example 6) and Chapter 8.

REL=“string” (optional; valid with HREF only; not currently supported) REL attribute values describe the relationship(s) associated with the hypertext link—consequently, REL is only valid if HREF is also present. The relationship is defined between the two entire documents and is not just associated with the particular link. As an example, REL could indicate that the linked document is an index for the current one or is an annotation to the current one (which a browser might want to display as a pop-up). It is a pity that the REL (and REV, which is the converse of REL) attribute is so little used, as it can impart significant meaning and organization to large sets of related documents.

The value for REL is a space-separated list of case-insensitive relationship values (preferably a collection of name tokens). Some examples are:

<A HREF="http://foo.edu/fe.html" REL="next">sdfsddf</a>

This would mean that the document fe.html, at the given URL, is the next document in some author-defined document sequence. Values for the relationships and their semantics are currently being defined. Some other examples are:

<A HREF="http://foo.edu/note1.html" REL="annotation">related notes</a>

The information in the document note1.html is additional and subsidiary to the current document. A browser might display this as margin notes.

<A HREF="http://foo.edu/vers2.html" REL="supersedes">previously</a>

The document vers2.html is an earlier version of the document.

At present, the REL and REV attributes are rarely used, and most browsers do not understand them. They will be of growing importance as HTML documents and document development environments become more sophisticated.

REV=“string” (optional; valid with HREF only; not currently supported) REV is like REL but with the relationship reversed. For example,

<A HREF="http://foo.edu/vers2.html" REV="supersedes"> later </a>

means that the document vers2.html is a later version of the document containing this link. Most browsers do not understand REV or REL.

SHAPE=“circle,” “default,” “poly,” “rect” (optional; used inside OBJECT only; If present, then COORDS is also mandatory; not currently supported) SHAPE is valid only for anchor elements within OBJECT elements, as discussed previously. SHAPE, in combination with the COORDS attribute, defines active regions of the image referenced by an OBJECT and attaches these regions to the remote resource indicated by the HREF attribute. SHAPE simply specifies the type of shape, the possible values being “circle,” “default,” “poly,” and “rect.”

TABINDEX=“number” (optional; Internet Explorer 4 only) Specifies the tabbing order for the linked region. By default, a user can use the tab key to switch between the various anchor elements, active images, and FORM input elements, starting from the top of the document and working down. TABINDEX lets an author change this default order and explicitly set the order of tabbing. This attribute is not supported on Netscape Navigator 4.

TABINDEX can apply to A, AREA, OBJECT, INPUT, SELECT, TEXTAREA, and BUTTON elements. The value assigned to TABINDEX is an integer: The smaller the integer, the higher the element is in the tabbing order. If two elements have the same TABINDEX value, then they are accessed in the order in which they appear in the document. Elements without a TABINDEX are accessed, sequentially, after all the elements with specified TABINDEX values. Elements with negative TABINDEX values are not accessed via tabbing, nor are any form input elements marked with a DISABLED attribute.

For example, consider the following elements and the associated TABINDEX values: The column to the right shows the order in which the elements will be accessed should the user employ the tab key to switch from element to element:

Element Markup	Tabbing Order
<A TABINDEX="5" ....> ...	3
<INPUT TABINDEX="3" ...>...	1
<INPUT TABINDX="23" ...> ...	5
<A HREF="foo.org"> ...	6
<A HREF="foo.org" TABINDEX="-1"> ...	not accessible via Tab
<SELECT TABINDEX="12" ...>...	4
<A TABINDEX="4" ...> ...	2