It totally does! Default hotkey “/” or from the menu: Help → Search and Run a Command

Well if you want modern CSS3-support or even JS you have to go for a browser. You could try NetSurf which is a very small & fast but only supports CSS2. Or Dillo, which is even smaller & faster (but also more limited in terms of CSS support)

The very basic version only allowing for one filter would be EasyEffects. If you want to use different filters for different programs you can install some LV2-plugins or bundles and then configure LV2-Plugins directly in pipewire (which is a bit of a pain) or use a GUI plugin-host like Calf and probably some patchbay (qpwgraph, patchmatrix, …)

Or even better: try Carla, it may be a exactly what you want and can be remote controlled via MIDI or OSC