The hallmark of any good IDE is its extensibility. If developers can’t improve their own tools then they won’t see productivity improvements over time. The same goes for web designers, which is why Microsoft’s Expression Web 4 now offers an add-in model based on HTML+JavaScript.
HTML and JavaScript was chosen because that is the language most web designers know the best. Extension points allow you to create menu items, toolbar buttons, panels and dialog boxes. It does require some XML to wire it into the IDE, which can be somewhat error prone and tedious.
Fortunately there is a tool called the “Add-in Builder” to help with that part. IT is available via the Expression Web blog. To further add in constructing add-ins, Erik Mikkelson has created a test harness that shows the outputs, errors, and events that arise from the extensions. He is referring to this tool as the Extensibility Tester.