What are the online GTD schedulers with support for projects and contexts?

Question

Time Management

I'm looking for a GTD scheduler. Key features:

• Cross-platform clients or web access
  
• Support for projects and contexts.
  
• Sequential activation of tasks in the project. (That is, if there is a project of three tasks, then only the first one is visible in the list of active tasks. After its completion, only the second one. Then the last one)
  
• Free if possible
  

Answer 1

With the sequential activation of tasks, I only know OmniFocus. But it's not online or cross-platform (Mac and iOS only).
All other GTD-like schedulers that I have seen (and I have seen quite a few) do not have the ability to hide tasks until the previous one is completed.

Answer 2

MLO:
- projects
- sequence of tasks (it is possible in order, it is possible with setting up individual dependencies)
- contexts
- not everything is smooth with cross-platform - there are Windows, iPhone, iPad, Android, it is being developed for Mac OS, under WP - plans are unclear, Web - no .