XML is nowadays widely used to describe structural documents on the WWW. The layout of the documents can be defined by style sheets or by a language itself. XML languages can be combined to get the desired set of features for a document. Those documents are called compound documents. Also, multimedia applications can be defined using XML. In this paper, we have defined general requirements for a compound documents? CSS layout engine, which can be used in an XML user agent. The requirements relate to features of the layout engine, devices in which it can operate, and interfaces of the cooperating components. In addition, we describe our own implementation of a layout engine, which is done according to the requirements and operates in different devices.