As was mentioned in some other reports, the HIG spacings are unusable
for programmers. Because of how Gtk+ layout is done, spacings not given
in terms of toolkit function arguments lead to inconsistencies. For
example, in bug #95773, it's made clear that though at one point the
HIG recommends 12px for a spacing, the function argument should be 6px
(maybe).

Neither designers nor programmers can make any use of spacings which are
given in terms of appearances on one person's screen. Various aspects of
themes will alter the visible result.

Gtk+ style properties (which are themable) that can be overridden
by code should default to HIG values. This way the overrides are
unnecessary, so global changes can be made easily. Without an
intelligible HIG, getting changes made to Gtk+ default will be almost
impossible.