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Declarative multimedia documents represent the description of multimedia applications in terms of media items and relationships among them. Relationships specify how media items are dynamically arranged in time and space during runtime. Although a declarative approach usually facilitates the authoring task, authors can still make mistakes due to incorrect use of language constructs or inconsistent or missing relationships in a document. In order to properly support multimedia application authoring, it is important to provide tools with validation capabilities. Document validation can indicate possible inconsistencies in a given document to an author so that it can be revised before deployment. Although very useful, multimedia validation tools are not often provided by authoring tools. This work proposes a multimedia validation approach that relies on a formal model called Simple Hyper- media Model (SHM). SHM is used for representing a document for the purpose of validation. An SHM document is validated using a hybrid approach based on two complementary techniques. The first one captures the document's spatio-temporal layout in terms of its state throughout its execution by means of a rewrite theory, and validation is performed through model-checking. The second one captures the document's layout in terms of intervals and event occurrences by means of Satisiability Modulo Theories (SMT) formulas, and validation is performed through SMT solving. Due to different characteristics of both approaches, each validation technique complements the other in terms of expressiveness of SHM and tests to be checked. We briefly present validation tools that use our approach. They were evaluated with real NCL documents and by usability tests. [ABSTRACT FROM AUTHOR]