Portal sites such as Yahoo and Excite remain basically cross-platform Web sites, written to offer their content and services to users running Microsoft's Windows operating system, the Macintosh platform, Unix, or other operating systems.
Enhanced features, however, are likely to run only on Windows.
Microsoft has pointed to the rise of Web-based applications as evidence that it faces robust competition in the software applications market, and that it does not have a permanent stranglehold on the operating systems market. The company has advanced this argument in its own defense in its current antitrust trial.
But if those Web-based applications increasingly rely on Windows for full functionality, Microsoft's argument could lose some of its strength.
As software developers helped make Windows a standard by porting nearly all of their work there first, providers of plug-ins are boosting Windows in the same manner. Generally, when a plug-in is released, it is released for Windows first, reinforcing the platform's popularity.
Portal giant Yahoo offers four utilities that provide full functionality only for Windows users. These are the TrueSync synchronization feature of Yahoo Calendar and Address Book, which requires a third-party software download only available for Windows; Yahoo Companion, a souped-up browser toolbar; Yahoo Radio, a streaming music service based on RealNetworks' cross-platform RealPlayer G2 but only available from Yahoo for Windows; and Yahoo Messenger--formerly called Yahoo Pager--an instant messaging application.
Mac and Unix users are provided a Messenger version written in Sun Microsystems' cross-platform Java programming language, but that version has limited functionality.
Analysts say the same market forces that propelled Microsoft's operating system and applications to dominance are still in play on the Web.
"In order to differentiate themselves, vendors find they need to deviate from standards," said Rob Enderle, analyst with the Giga Information Group. "With regard to the Web, that means you may need to download and run something. If so, the vendor is going to design for the most prevalent operating system. The market trends that created Microsoft and Windows are still in effect. They have not gone away."
Yahoo is not the only portal offering enhanced functionality for Windows users. Excite@Home's Voice Chat requires a Windows plug-in. The portal's Excite Planner calendar also requires a Windows download for synchronization, as does its Excite Assistant, a separate window with personalized information, email alerts, and a radio dial.
"We come from a long tradition of being multiplatform," said Netcenter spokesman Nate Tyler. "It's a high priority here. As long as you have a browser, you can access any service on Netcenter."
Yahoo defended its use of Windows-only plug-ins, saying that the portal has to offer functionality tailored to emerging technologies, even if only for a single platform. One such example is the synchronization feature of the calendar, which lets users of personal digital assistants (PDAs) synch up their Web-based information with what's on their handheld device.
"Are we a Microsoft shop? Absolutely not," said Henry Sohn, Yahoo's director of production for personalization and membership. "Our core tools we will offer across the board. But certain platforms have requirements.
"Today, not very many people surf the Web via PDAs or PalmPilots. But that could change, and if it happens, it probably makes sense for us to do some development for that platform," he added.
Sohn noted that the age of Web-based applications is even more complicated for developers than the traditional desktop software era. Now, developers have to take into consideration not only the operating systems but users' browsers as well.
Developers have the option of creating all their tools to be cross-platform, but cost of development and efficiency of the final product make that unlikely for cutting-edge tools and technologies.
Those developers face similar problems as those working with Java, according to Enderle.
"Java never rose to its full write-once-run-anywhere potential, because it generically runs more poorly than something optimized for a platform," he said. "The same thing happens for Web technologies. People optimize for where the audience is, not for where they're going.
"If you truly wanted to develop something that was OS-independent, you would have to put a lot of work into it, write for OSes that aren't in widespread use, and anticipate OSes that aren't out there yet," he added.