How to make a similar hole for a button?

Question

css

I remember a similar topic, but lost the link. Using CSS

Answer 1

The code is very unkempt , but the result is working.

Answer 2

Hmm .. in my opinion, is this the border of the button just for the color and with the transparency of the left block?

Answer 3

codepen.io/vlad7576/pen/NxvvEQ If you want the block to be slightly transparent, but not :after, set rgba