The XML FAQ

Mike Brown writes:

The concept of ‘browsing’ is primarily the result of HTML having the semantics that it does. In an HTML document there are sections of text called anchors that are ‘hyperlinked’ to other documents that might be at remote locations on a network or filesystem. HTML documents provide cues to a web browser regarding how the document should be displayed and what kind of behaviors are expected of the browser when the user interacts with it. The HTML specification provides many suggestions and requirements for the browser, and provides specific meanings for many different examples of markup, such as the fact that an <img> element refers to an image that should be retrieved by the browser and rendered inline with the adjacent text.

Unlike HTML, XML does not have such inherent semantics at all. There is no prescribed method for rendering XML documents. Therefore, what it means to ‘browse’ XML is open to interpretation. For example, an XML document describing the characteristics of a machine part does not carry any information about how that information should be presented to a user. An application is free to use the data to produce an image of the part, generate a formatted text listing of the information, display the XML document's markup with a pretty color scheme, or restructure the data into a format for storage in a database, transmission over a network, or input to another program.

However, despite the fact that XML documents are purely descriptive data files, it is possible to ‘browse’ them in a sense, by rendering them with stylesheets. A stylesheet is a separate document that provides hints and algorithms for rendering or transforming the data in the XML document. HTML users may be familiar with Cascading Style Sheets (CSS). The CSS stylesheet language is general and powerful enough to be applied to XML documents, although it is oriented toward visual rendering of the document and does not allow for complex processing of the document's data. By associating an XML document with a CSS stylesheet, it may be possible to load an XML document in a CSS-aware web browser, and the browser may be able to provide some kind of rendering of it, even if the browser does not otherwise know how to read and process XML documents. However, not all web browsers will load an XML document correctly, and they are not required to recognize the XML markup that associates the document with a stylesheet, so one cannot assume that XML documents can be opened with just any web browser.

A more complex and powerful question C.24, stylesheet language is XSLT, the Transformations part of the Extensible Stylesheet Language, which can be used to transform XML to other formats, including HTML, other forms of XML, and plain text. If the output of this transformation is HTML, it can be viewed in a web browser as any other HTML document would.

The degree of support for XML and stylesheets in web browsers varies greatly. Although loading and rendering XML in the browser is possible in some cases, it is not universally supported. Therefore, much XML content on the web is translated to HTML on the servers. It is this generated HTML that is delivered to the browsers. Most of Microsoft's web site, for example, exists as XML that is converted to HTML on the fly. The web browser never knows the difference.