Metadata Standards and Web Services in Libraries, Archives, and Museums by Erik Mitchell

Author:Erik Mitchell
Language: eng
Format: epub
Publisher: ABC-CLIO
Published: 2015-12-15T00:00:00+00:00

Standard Generalized Markup Language (SGML) and its Descendants

The Standard Generalized Markup Language (SGML) is a declarative markup language that was originally designed to mark up large documents so they could be shared via computer. Because SGML is declarative, it should describe a document’s structure and other attributes and not specify the kind of processing that should be performed on it. SGML is significant as a milestone in the history of computer and software, but not because of its existence per se, but rather because two of its descendants are HTML and XML.

The HyperText Markup Language (HTML), as we know from Chapter 2, is the markup language used by web browsers to define web pages. Originally, HTML was intended to contain all the information that a web page would need. Later, support for images, JavaScript, and embedded objects such as videos was added, eventually turning web browsers into very sophisticated tools—allowing them to become computing platforms in their own right, in many ways independent from the operating systems that host them. With the introduction of JavaScript and Cascading Stylesheets (CSS), our thinking about web applications, web pages, and metadata abruptly changed. Before these developments, web pages existed as complete entities where each displayable HTML element had its default rendering and formatting characteristics chosen by the browser. To override these, the web page author added attributes to the elements to, for example, change font size, type, or color. Afterwards the Model-View-Controller (MVC) paradigm came into use in web browsers, where the “model” was a hierarchical data structure representing the HTML called the Document Object Model (DOM); the “view” was metadata in the form of a series of declarations in the CSS stylesheets, which defined how each HTML element ought to be rendered by the browser; and the “controller” was any JavaScript code executed in response to users’ mouse clicks and key presses. HTML was revolutionary in another aspect in that it changed how we think of data and its serialization. Modern web applications regularly use some form of the Asynchronous JavaScript+XML (AJAX) interrelated technologies in JavaScript programs to retrieve information from web servers, parse it, and then update the web page’s DOM. Originally, the data returned from the server used XML format. More often, JavaScript Object Notation (JSON) is used. We will talk more about this serialization format later in this chapter.

The eXtensible Markup Language (XML) is a simplified subset of SGML, containing most of its important features. However, SGML has been subsumed by XML, as it can be used for the purpose SGML was originally devised. XML, like all markup languages derived from SGML, is hierarchical in nature. Unlike HTML, however, it must be fully formed (i.e., every open tag must have an end tag) in order for an XML document to be parsed correctly. Web browsers, on the other hand, implement document fault tolerance and graceful degradation—rendering only what is supported, skipping over what is not, and attempting to render something that is not syntactically correct or malformed. This has led to some malformed HTML elements becoming acceptable and eventually part of the standard.

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