OmniGroup has opened enrollment for beta testers for its OmniFocus web app that will be opened for testing "later this year". You need an OmniSync account. Free. OmniGroup's proprietary cross-platform sync service for its products. Log into OmniSync on the web, scroll down to options and enroll in the OmniFocus web beta. I believe a copy of OmniFocus 3 for iOS is also required, so I wouldn't buy that just to play with the beta - but if you have it, enrollment in the web beta is available.

https://manage.sync.omnigroup.com

Little bit of extra info on the Omni blog - second item

https://www.omnigroup.com/blog