This paper addresses the issue of Web document summarization. As textual content of Web documents is often scarce or irrelevant and existing summarization techniques are based on it, many Web pages and websites cannot be suitably summarized. We consider the context of a Web document by the textual content of all the documents linking to it. To summarize a target Web document, a context-based summarizer has to perform a preprocessing task, during which it will be decided which pieces of information in the source documents are relevant to the content of the target. Then a context-based summarizer faces two issues: first, the selected elements may partially deal with the topic of the target, second they may be related to the target and yet not contain any clues about the content of the target.In this paper we put forward two new summarization by context algorithms. The first one uses both the content and the context of the document and the second one is based only on the elements of the context. It is shown that summaries taking into account the context are usually much more relevant than those made only from the content of the target document. Optimal conditions of the proposed algorithms with respect to the sizes of the content and the context of the document to summarize are studied.