The Text Encoding Initiative (TEI) is a consortium that developed the standard for the representation of digital texts. The standard is widely spread, and currently in use by publishers, libraries and museums.

Like a lot of standards for texts, TEI is also in XML. The TEI provides XML Schema’s for validating documents. It is very important to produce valid documents. Only when all organizations use the same XML Schema, the documents will be interoperal between them.

On the website of TEI (http://www.tei-c.org/Tools/Stylesheets/) a lot of stylesheets are provided. These could be used for transformation of a TEI document into another type, like ePub, docx or HTML.