Abstract: Modularity is a key feature at design, programming, proving, testing, and maintenance time, as well as a must for reusability. Most languages and systems provide built-in facilities for encapsulation, importation or parameterization. Nevertheless there exists also languages, like HTML, with poor support for modularization. A natural idea is therefore to provide generic modularization primitives. To extend an existing language with additional and possibly formal capabilities, the notion of \textit{anchorage} and \textit{Formal Island} has been introduced recently. TOM for example, provides generic matching, rewriting and strategy extensions to JAVA and~C. In this paper, we show on the HTML example, how to add modular features by anchoring modularization primitives in HTML. This allows one to write modular HTML descriptions, therefore facilitating their design, reusability, and maintenance, as well as providing an important step towards HTML validity checking.