For those of us who have an existing application that supports multiple LAFs, and don't want to rewrite the whole app to use WebLookAndFeel-isms, it would be nice if a "close enough" border for some components (e.g. TextField, TextArea, etc.) were put into the UIDefaults table.
I understand the mechanism for painting borders is different for WebLaF, and so you wouldn't be able to provide something that would look "right" (e.g. it wouldn't highlight when focused, etc.). However, I sometimes have custom components that steal existing components' borders to blend better with whatever LAF is installed. For example, a breadcrumb bar, similar to that found in Windows Explorer. It uses TextField.border. In the case of WebLaF, this property isn't defined, so we fall back to the BasicLookAndFeel's TextField.border, which is the ugly old beveled style. If even a LineBorder of the "right" color were installed, it would go a long way for custom components.
I'm not above writing an adapter class that installs & tweaks WebLookAndFeel via reflection if it's installed by the user, so alternatively, if there's some way to "install" a standard WebLaF Painter on a component to render its border-ish parts, I'd be happy to do that as well if you pointed me in the right direction. I'm currently exploring the StyleConstants class and maybe I'll figure something out.