In human communication, mutual understanding of meaning is often achieved through an iterative process. Our common experiences provide a platform on which we can express our thoughts and opinions. On matters of fact, we can exchange evidence until we reach consensus. On matters of opinion, we can exchange views until we reach understanding.

Communication with and between machines does not generally allow the luxury of partial understanding. Machines have not yet demonstrated any ability to proceed in the face of ambiguity. On the Web, where communication is achived by the exchange of resources (most often documents in a broad sense, but generally any resource that is identified by a URI can be exchanged), we need a clear means by which meaning can be associated unambiguiously with a document.

A document on the Web is a stream of bits identified with a specific MIME type. The MIME type indicates to the processor how it may interpret the stream of bits to decompose it into a sequence of characters, for example, or a specific bitmap image.

One of the fundamental benefits of an open, extensible language like XML is that it makes information reuse very cheap. A number of mechanisms exist that allow me to quickly and efficiently repurpose information on demand.

Some forms of reuse are easy to predict: publishing information for multiple platforms and multiple hardware architectures, converting between application formats. Others are harder to predict: transforming legacy information for new systems, mining existing data resources for new information, and applications net yet imagined.

It is important to preserve this cheap, flexible reuse policy as we develop the Web architecture.

On the Web, every document has a MIME type in addition to the stream of bits that comprises its content. In the absense of a MIME type, a stream of bits is essentially meaningless. It's the MIME type that tells a process how it can interpret the stream of bits. Although good engineering often encourages us to design self-describing data formats, it's quite possible for the same stream of bits to have different semantics depending on the MIME type associated with it.

Consider an XHTML document. If it is served as text/xhtml, the receipient is obligated to process it as XML, performing well-formedness and perhaps validity checking, namespace processing, and a small number of other tasks. If the same document is served as text/html, the interpretation of those bits is governed by the HTML Recommendation which has no provision for XML namespace processing or well-formedness.

So, a first level approximation of a document's meaning is determined solely by its MIME type.