I am not an expert of the topic, but latency of different computer systems has to be accounted for. Of course it depends on whether the content is visual, but the latency of LCD screens is a very important factor. In older LCD monitors, latency can reach up to 18ms and depends on the displayed material/motion as well, whereas newer, gaming LCDs can go down to 2-4ms. This means that theoretically people with better screens will see information appear (or disappear) 14ms sooner.

Another factor is quality of video card. If the card is slow, there will be "tearing" in material requiring the screen to refresh quickly. Tearing means that there is visible delay between the refreshment of the upper and lower part of the image:

https://www.ultimateeditionoz.com/forum/download/file.php?id=10142&mode=view

Obviously, this can be a significant problem, if you need to look at the screen as a whole.

The last, and maybe most obvious factor is screen size.

Considering the above, it is best to use the same system (at least displays) to generate comparable data among subjects.

David,

thank you for very thoughtful comments. You brought up very important technical issues with online tools.

Andrew,

I will go with an offline software, just to avoid the extra noises which will be too hard to control when using an online system. I agree, Eprime is a useful tool to developed my own software. Thank you again for taking your time to reply my question. Best, Nuray