I've been writing a new web "framework", aka library, aka toolkit, and I've decided to use the GPL for it. In my interpretation of the GPL, this will require plugins and libraries which are on top of the framework and distributed with it to also be licensed under the GPL.
The important key word there is distributed. The GPL is a strong "copyleft" license for most locally run software, meaning it has to be distributed to be used, but for web applications, the license isn't as extreme. That's why I've decided to use it for Schematronic.