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Use a tool like the "Snipping Tool" in MS Windows to take a screen capture from one of the WebGL Google Chrome Experiments from here:

https://experiments.withgoogle.com/chrome?tag=WebGL

Make sure you leave a link to the overview page for the experiment you choose. There is a huge list of demonstrations to choose from.

Is there a special label?

Do you mean tag? This is my first contest I'm hosting. Is that how they are typically run? I was planning on having entrants just comment with their entries.

I participated in one where it was labeled,
Where would you place the entrance? How would the title?

I'm just going to do this contest with entries as replies to this original post with a screen capture and link in it.

I understand, I'm going to participate