Cathal and Patja are both right, but about different aspects.
For viewing a DNN site as a web visitor, it comes down to the skin. You can make DNN work just as well as anything else in that regard.
But as Patja points out, the currently included FCK editor just doesn't work in Safari or even in Opera, and it almost certainly won't work in those old browser versions you've listed... Netscape 4 should definitely be on the scrap heap.
So for your site visitors, yes it'll work, but for your site publishers, no it won't - And if you're using a forum or other module that provides the FCK editor, then your users will have to use Firefox or IE.
The next release of the editor will support Safari and Opera, but likely only new versions of the browsers, not old ones.
Now.. does anyone have a cool script I can put into the Text/HTML module to alert users who have signed in with Opera or Safari that they should switch to Firefox?
Rob