In this paper we outline a multimodal interaction description language XISL, (Extensible Interaction-Sheet Language), that is developed to describe multimodal interactions (MMI), and to make the description of interactions independent of XML contents. XISL makes an XML document independent of interactions that may differ between each terminal, and so enables such seamless services as web-browsing to be constructed easily. Since XISL is also independent of the devices, and has the flexibility to describe modalities, the system developer can easily change the user interface without introducing a new set of description language. We implemented an interpreter of XISL, and proved the viability of XISL by experiments using an online shopping application described by XISL.