What technologies to choose for a multiplatform slideshow service?

Question

JavaScript

I'm planning to start developing a new service. The server part will be, but extremely simple, mainly download, save content. The client part should work both through the browser (as the worst option is a desktop program) and on mobile devices and use a rather complex UI (timeline, complex lists), as well as animation, effects, particles, video. Kind of like a slideshow builder.
It's easiest for me to do everything in Flash, but recent trends (browser support for Flash, infrequent AIR updates) hint that this can be a risky investment.
What else would you recommend for cross-platform development? There is one code in the plan for both Android && iOS and web || desktop and with a minimum of glitches, crashes, performance losses. With ready-made UI components, developed community. And, preferably, free tools.

Answer 1

After the words "animation, effects, particles" Unity immediately comes to mind.