Way, way back in the early days when we were planning IP.Board 3, a primary consideration was to completely overhaul the output engine to add several new features and to increase extensibility.
Out with the old...
The system in IP.Board 2.x is really just a perfunctory "engine" build around a few methods in a class. There was no real cohesive structure with many different files and functions accessing 'skin' methods. We decided that a virtual re-write was required.
We didn't want to be tied to a single output format. Back when IP.Board 2 was first written, there were no iPhones and the idea of actually viewing a forum on a cell phone seemed more than a little crazy. Times have changed.
...and in with the new
At first glance, the new system isn't that different from what we had in IP.Board 2. It's when you scratch the surface that its power becomes more obvious.
The first change is that you can have an unlimited depth for child skins. In IP.Board 2 you could only have one level of child skins which was very limiting to some skinners. Also, in IP.Board 2 there was a single master skin from which each skin inherited from.
While there is still a single "master" skin in IP.Board 3.0.0, it's completely transparent and instead you can set up several different "root" skins for which child skins inherit from. This instantly makes for more flexibility.
In IP.Board 3, each skin can have multiple CSS files which can be hard-positioned within the ACP to ensure the correct load order for correct cascading.
Also, each skin allows you to set group-based permissions so that you can determine exactly who can view and select skins.
And finally, guests can select a skin (as long as you give them permission to do so!)
Going Deeper: User Agents
IP.Board has a completely new user-agent system where you can add new user-agents and group together several. This system is now used for the 'search engine spider' settings.
This also means that you can tie a user agent to a specific skin. Do you want your iPod touch and iPhone users to use a specific skin? You can do that very easily within the ACP. Taking it further, you can even designate specific versions for each user agent. Do you want to take advantage of Firefox 3 or IE 8? That's very simple to do now. You can even set a range of versions very simply.
Going Deeper Still: Output Formats
The biggest change is that IP.Board can now handle multiple output formats. By this we mean that IP.Board has a plug-in architecture to allow completely different processing for HTML, XML or even WAP. This system is completely extensible so modification authors can easily write their own engine plug-ins which drop in and are available for use immediately.
Each skin must choose an output format to use which means that you can have completely different skin sets for XML and HTML bringing in even more flexbility.
You can also use a "gateway" file to set the output format. IP.Board 3 ships with two gateway files. "index.php" which is tied to the HTML output format and "xml.php" which is tied to the XML output format. This means that "index.php?showforum=1" and "xml.php?showforum=1" enable the correct output engine instantly.
Putting it Together
Take this scenario: You want to support WAP using XML with XLST templates specifically for Nokia cell phones. Done, done and done. All without having to make a single PHP modification.
Now that's a powerful system!