em
Emphasizes a text element. 

In the Microsoft Reader, it renders like the <i> tag.

attributes

id
Provides an element 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 one or more classes.
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.