Steam violates ICCCM in at least the following two ways:

ICCCM 4.1.7. Input Focus

Passive Input − The client expects keyboard input but never explicitly sets the input focus.

Steam indicates on its top-level windows that it follows the Passive Input model. (Input field: True, WM_TAKE_FOCUS absent) yet it calls SetInputFocus.

ICCCM 4.2.4. Window Resize

The response of the client to being resized should be to accept the size it has been given and to do
its best with it. Clients must not respond to being resized by attempting to resize themselves to a
better size.

If the window manager resizes a steam window to anything less than the minimums specified in WM_NORMAL_HINTS, Steam continuously tries to resize the window.

Related Issues:
#807
#1301

Steam client application
Built: Oct 16 2015, at 12:09:15
Steam API: v017
Steam package versions: 1445025047