Abstract: This page summarizes the intents of logical and physical markup, and argues for the use of logical markup.
The World-Wide Web, like many hypertext systems, is based upon a markup language, in which authors annotate text to describe the texts role or appearance. There are two different views of markup, logical markup and physical markup. HTML has some logical tags and some physical tags.
In logical markup tags describe the role a piece of text plays. For example,
<em> indicates that "this should be emphasized".
<h2> indicates that "this is a section header".
<li> indicates that "this is an item in a list".
Physical markup describes the appearance of a piece of text. For examle,
<it> indicates that "this should be italicized"
<center> indicates that "this should be centered"
<text color="blue"> indicates that "this text should be blue"
As you might guess, there is a lot of discussion (argument?) as to which type of markup is "better". When designing a web page, it is important to know what kind of markup you are using, and why you have chosen to use that type of markup. I generally advocate logical markup, with a little bit of physical markup when appropriate to enhance readability.
There are a number of reasons to support logical markup. Some benefits accure to the author of a logically-marked text, others to the readers. The simplest reason to use logical markup is that it makes you think about the text you are writing: "What is the role of this component? How should I indicate that to the reader?"
Logical markup also empowers readers. Many browsers permit readers to customize the way logically marked text is displayed. In some browsers, you can say "show emphasized text this way, show headers this way, ...". You may have encountered a simpler version in the font and color settings of Netscape. By using logical markup, you give your readers the option to describe how their text is displayed.
This gives you two immediate advantages:
Logical markup can make it possible to distinguish between similar looking pieces of text with diverse meanings. For example, both book titles and emphasized text are usually put in italics, but for very different reasons.
Finally, logical markup better supports a mixed group of users. For example, the color-deficient (or those with only b&w monitors) may have trouble reading a document with many color changes. Similarly, a "reader" for visually-impaired users might better be able to interpret logical tags than physical ones (e.g., for emphasis, it might get louder). T.V. Raman has written some excellent research and opinion papers on this issue (one of which was in a recent issue of Scientific American).
Unfortunately, HTML only supports a fraction of the possible logical markup tags, although it may be that it will soon allow you to define your own.
There are also some good arguments for physical markup. Primary among these is that traditional designers spend a lot of time getting their designs to "look right". All of that knowledge and effort is lost if only logical design is available.
It's also clear that you end up doing some "design" (such as ordering of elements), even if you don't rely on physical design. You can use physical design to better present some elements, such as tables of contents.
This page written by Samuel A. Rebelsky.
This page generated on 55 by SamR's Site Suite.