Résumé : Microformats and semantic XHTML add semantics to web pages while taking advantage of the existing (X)HTML infrastructure. This approach enables new applications that can be deployed smoothly on the web. But there is currently no way to describe rigorously this type of markup and authors of web pages have very little help for creating and encoding semantic markup. A language that addresses these issues is presented in this paper. Its role is to specify semantically rich XML languages in terms of other XML languages, such as XHTML. The language is versatile enough to represent templates that can capture the overall structure of large documents as well as the fine details of a microformat. It is supported by an editing tool for producing documents encoded in a semantically rich markup language, still fully compatible with XHTML.