Abstract : Eliom is a dialect of OCaml for Web programming in which server and client pieces of code can be mixed in the same file using syntactic annotations. This allows to build a whole application as a single distributed program, in which it is possible to define in a composable way reusable widgets with both server and client behaviors. Our language also enables simple and type-safe communication. Eliom matches the specificities of the Web by allowing the programmer to interleave client and server code while maintaining efficient one-way server-to-client communication. The Eliom language is both sufficiently small to be implemented on top of an existing language and sufficiently powerful to allow expressing many idioms of Web programming. In this paper, we present a formalization of the core language of Eliom. We provide a type system, the execution model and a compilation scheme.