Fluid has several extension points that developers (and power users) can leverage.
Bookmarklets. Fluid Apps feature a handy Bookmark Bar UI similar to Safari and other browsers. Not surprisingly, Bookmarklets can be placed in the Bookmark Bar for basic scripting features.
Userscripting. Fluid Apps come with Userscripting built-in. No SMBL Plug-ins (or any other type of Plug-in) are used or required. You can manage your Fluid App's Userscripts via the Main Menu: Window → Userscripts.
Userstyles. Fluid Apps come with Userstyles also built-in. You can manage your Fluid App's Userstyles via the Main Menu: Window → Userstyles.
window.fluid.dockBadge = "10"
window.fluid.requestUserAttention() // bounce Dock icon
window.fluid.beep() // sounds system beep
window.fluid.playSound("Basso") // plays system sound if name is valid