This term encompasses any of a number of devices and device types. It is basically any device or technology that uses electronic means to transmit and present information to the user. Examples include Computers, PDAs, cell phones, information kiosks, televisions, and many other devices.

In XHTML, an element is of node in the document tree. A XHTML element is represented by a start tag like <h1> and an end tag like </h1>.

Content is "equivalent" to other content when both fulfill essentially the same function or purpose upon presentation to the user. In the context of this document, the equivalent must fulfill essentially the same function for the person with a disability (at least insofar as is feasible, given the nature of the disability and the state of technology), as the primary content does for the person without any disability.

The box around a font character of a given 'size' as designed by the designer. It is sometimes referred to as an em-square. The theoretical square that glyphs are designed upon. Its height is the intended distance between lines of type in the same type size. It is the design grid on which the glyph outlines are defined.

In CSS, an' em' is what is referred to as a relative unit, like percentages. Good design allows scaling and uses relative size values; it almost always provides excellent implementation of ems. An 'em' is equal to the computed value of the 'font-size' property of the element on which it is based. The exception is when an 'em' occurs in the value of the 'font-size' property itself, in which case it refers to the font size of the parent element. It may be used for vertical or horizontal measurement.

An embedded style controls the appearance of a single document. CSS rule(s) are placed in the head of the document via the style element. Embedded styles can be really helpful when you are first learning to code CSS, because it is very easy to edit and troubleshoot the code. Similar to inline styles, embedded CSS defeats the purpose of separating content and presentation. An embedded style sheet can be useful when a single document has a unique style. If the same style sheet is used in multiple documents, then an external style sheet would be more appropriate. Embedded styles are sometimes referred to as internal styles.

An external type of style sheet is a separate file apart from the HTML file(s). The external style sheet is given a .css extension. It is associated to the HTML document using a link element or an @ import directive in the head of the html document. Any html document associated to an external style sheet gets the styles, and that is where the maintenance power of CSS is found. Not only do external style sheets allow reuse of rules within one specific document. They allow the style sheet itself to be shared across multiple documents. Using external CSS allows you to maintain a single style sheet for an entire site.

An eyetracking device observes a person's pupil to determine the direction of their gaze. It can aid in learning the relative intensity of a user's attention to various parts of a web page.