The HTML5 client should support zoom. Check if upstream has any plans for this.
There seems to be some work done on the subject here: https://github.com/kanaka/noVNC/issues/12 It's very old though, I will investigate and try it.
On touch devices zooming could be implemented to work when pinching with two fingers. This would be especially useful on touch devices with small screens.
*** This bug has been marked as a duplicate of bug 7356 ***