CSS (Cascading Style Sheets) is a language used to describe the presentation of a web page. While various HTML tags and attributes provide similar functionality, modern web page design practices encourage HTML for content and CSS for presentation. A web page can have many style sheets whose effects are brought into play based on user interaction, browser type, and a variety of other factors. From a testing perspective, this can create a challenging environment as presentation and layout can change dramatically based upon the currently selected set of CSS characteristics. For more reading, go to Wikipedia or CSS at the Open Directory Project.
The DOM (Document Object Model) is a set of scriptable entities maintained by a browser that allow you to programmatically manipulate both the HTML and CSS characteristics of the page. The DOM represents most HTML tag attributes as object properties. It also defines methods that simulate user interaction, such as submitting a form, as well as an event model for triggering actions based on user interaction. While the browser DOM is embodied in a W3C standard, there are gaps and inconsistencies in support across popular browsers, not to mention various browser-specific extensions to the DOM. This is another compelling reason to do cross-browser compatibility testing. See Wikipedia or W3.org for more.
JavaScript is a scripting language supported by all browsers. It is used to programmatically change the state of the DOM. Although the allowable syntax for JavaScript is well defined in ECMAScript, writing complex client-side programs that manipulate the DOM entails the same potential for errors as any other complex coding effort. Testing JavaScript’s interactions with the DOM requires a well thought out, structured test effort. For more info, see Wikipedia or Mozilla's JavaScript documentation.
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