I won't claim Pax is better than any of these, but that we seek to solve different goals.

Unlike Flash: driven by a markup language, fully open source, no plugin required, solves a11y, compiles to native apps incl. mobile

Unlike Silverlight: fully open source, no plugin required, no heavy VM

Unlike JavaFX: no plugin required, no heavy VM, visual builder is a vector design tool

Unlike Flutter: designed for web (small footprint, a11y out of the box); first-party and foundationally integrated visual builder, visual builder is a vector design tool