How to change rotation point?

Question

In my next code I'll have to rotate an object, so wrote "transform:rotate(45deg)". The problem is that the position of the left corner mustn't change.
(The object is a rectangle that should look like a stroke. "45deg" is the corner and can be changed.)

Answer 1

Maybe this post can help you out - https://www.sololearn.com/Discuss/1550617/?ref=app