Problem to position my buttons in a three.js scene

Questions
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1

Hi,

I don’t know how to place my buttons with inline-block 50% vertically from my scene.
My example doesn’t work with percentage, but it works when I specify a pixel. I still specified “relative”.

Could you tell me what I am doing wrong in my html?

It’s not just a question about three.js but it will help me a lot for the future.

My codepen :

Thanks.