Extensions: Love 'Em!, Hate 'Em!
HTML is standardized so that any Web browser can read any Web documents ” sort of.
Here's the deal: All modern browsers support all of HTML 4, a well-established set of tags set by the committees that oversee Internet standards. Standardization is good because it provides Web authors with a way to ensure that most browsers can read what they publish. Because any browser can understand and interpret all the HTML 4 tags, authors need only stick within the confines of those tags to ensure that their pages are accessible to the biggest possible online audience.
The problem with standards, though, is that they evolve slowly. On the Web, only downloads are permitted to be slow; evolution is required to be fast. Think about it: The first graphical browser emerged seven years ago, and now we're talking real-time video. The entire birth and maturation of the Web as a graphical, interactive environment took place within the equivalent of a single Presidential administration. Yikes!
Leading browsers, including both Netscape and Internet Explorer, support all of HTML 4, the current standard. Still, the pace of Web page enhancement is so great that both Netscape and Microsoft continue to incorporate in their browsers extra tags and other capabilities that are not part of any approved HTML standard. These additional tags are extensions.
An extension is an HTML tag that makes possible some new capability in a Web page but is not yet part of the formal HTML standard.
The effects of these extensions, when used in a Web document, can usually be seen only through a browser that specifically supports them. Of course, Navigator supports many Netscape extensions, and Internet Explorer supports many Microsoft extensions. But subtle differences exist. For example, scrolling text banners that can appear in a box in the page layout in Internet Explorer appear instead in the status bar in some versions of Netscape. However, not all browsers support all extensions. That's why you need to be careful with 'em.
In general, whenever an incompatible browser accesses a page that uses these tags, nothing dire happens. The fancy extension-based formatting doesn't show up, but the meat of the page ”its text and graphics ” remain readable.