Yes it is. Competing with Microsoft I wouldn't go into direct competition with Microsoft. I wouldn't try to replace the primary web browser, at least not at this time. But I do site development software, and being able to preview a page as it will look in a web browser is an essential part of using my software. Of course it makes sense to build on the new HTML rendering features in Windows 98.
Risk is everywhere in business.
You can't avoid it, you have to embrace it. The Mac OS was much stronger two years ago. Since then, two billion dollars in Apple losses, a radical change in management and direction, and disappearing confidence among users and developers. There's probably still a good reason for Netscape to add emphasis to their Mac direction, but they'll find there are fewer people to work with in the Mac world now, two years later. You have to think like your competitor to be successful in a software world containing Microsoft.