Dear Thijme,
running Expyriment experiments online is not possible, as this would also have a pretty big impact on timing accuracy, which is a strong focus of Expyriment. We do have an Android app, though, but this is based on a rather old Expyriment version (0.7.0) and even there, visual timing accuracy is limited (due to not being able to block on the vertical retrace). One other option I heard from one of our users was to create a simple tutorial for participants to install Expyriment on their computers, use the test suite to test and record timing accuracy on each system, and let the user manually send data back. This obviously does not work for every participants as it requires (a) the willingness to install something on their local machine, and (b) some technical knowledge to do so. We can look if/how we can simplify this process even more from our side if that helps.
Best,
Florian