a
Specifies anchor point (href) or target point (name or id) for hyperlinking within a eBook. The destination of a link is specified as a URI. It is invalid to nest anchor tags. By default, links are presented underlined and displayed in colored text, but you can override this rendering using CSS properties.

Note: In Microsoft Reader,  external links are displayed as non-underlined text, and are not functional.

attributes

href
Specifies the destination URI. For instance, href="#targetpoint" set link to anchor named targetpoint.
title
Specifies a brief text string identifying the link.
id
Provides an tag with a unique name within a single document. This is especially useful in works with extensive cross-referencing and linking, such as dictionaries, encyclopedias, or travel guides. Use of ids simplifies linking and updates.
class
Affiliates an element with CSS class altering color and text-decoration properties.
name
For example,<a name="targetpoint"></a> specifies anchor point named targetpoint. 
rel
Provides information regarding the relationship of the item being linked to. This attribute is ignored.
rev
Specifies the relationship of this document to the document being linked to. This attribute is ignored.
style
Attaches a list of inline (name:value paired) CSS properties to a specific tag, for example: style="name:value;name:value"
Note: Media types other than "text/x-oeb1-css" will be ignored with a warning in a document of type "text/x-oeb1-document".
title
This attribute is ignored.