External EntitiesExternal entities are data elements defined outside the context of a formal definition environment. For example, the Netscape extensions are external entities to HTML 2.0 DTD. Many times external entities are created in order to add functions or options to an environment which were left out or overlooked in the original programming. However, external entities usually violate one or more of the environments strict rules of inclusion and are not supported by the authors of the original environment.
When designing a Web site, you should carefully consider whether or not to use external entities in your code. The external entities will add many features and layout options not available in the current DTD standard, but by using them you will be excluding those users whose browsers do not support the external entities. While Netscape is one of the more popular browsers and their associated extensions are widely used--you would still exclude 25% or more of the browsers which do not support Netscape extensions. As a good rule of thumb, using external entities should be done with caution. After you create a Web site, thoroughly check it with browsers which do not support the external entities in use. This will give you the perspective surfers will have while accessing your site. If you think the resulting look and layout is poor, try re-coding your pages so that browsers without external entity support will display your documents in a satisfactory manner.