Model-theoretic semantics is a formal account of the interpretations of legitimate expressions of a language. It is increasingly being used to provide Web markup languages with well-defined semantics. But a discussion of its roles and limitations for the Semantic Web has not yet received a coherent and detailed treatment. This paper takes the first steps towards such a treatment. The major result is an introductory explication of key ideas that are usually only implicit in existing accounts of semantics for the Web. References to more detailed accounts of these ideas are also provided. The benefit of this explication is increased awareness among Web users of some important issues inherent in using model-theoretic semantics for Web markup languages.