We support all modern/evergreen browsers.
If there’s a strong need/request, we could extend that to include IE11 (there are some issues with 3D transforms and SVGs in IE).
A fallback could be handled with user agent checks in your website (e.g. if modern browser, show haiku; else, show something else).
Any discrepancy between [Preview] mode in Haiku, and a published Haiku on share.haiku.ai should be rare— if there is, it’s likely browser related. Send us a link and we can look into it.