Concurrence of the Altitudes As Seen from 3D
The tool bar at the left is fully functional. Selecting the second button from the bottom allows you to rotate the diagram. Also, the vertices of the triangle are draggable such that you can modify the configuration and then view it from a different perspective.
In case GeoGebra does not load, here's a suggestive diagram of how the intersection of the altitudes is seen from 3D:
Copyright © 1996-2017 Alexander Bogomolny