[css-customproperties] is this good or bad?

While trying to figure out some incredibly perplexing treatment of some
inline <style> rules in ie7 I did a search and accidentally came across
this:

http://msdn.microsoft.com/en-us/library/bb250496(v=vs.85).aspx

Which uses the verbiage "custom properties" ....

"""
_ Underscore Filter
This CSS filter is based on a parser bug. It used to show properties
exclusive to Internet Explorer. These constructs will now be treated as a
"custom property" by Internet Explorer 7 and later. Custom property means
that it is still in the OM and can be queried through script but does not
natively apply its value.
/* The following rule used to apply min-height
   to browser who understand this property and
   height to IE.  In IE7, _height will be treated
   as a custom property (no height will be applied) */

.myclass {
   min-height: 300px;
   _height: 300px;
   ...
   }
"""

Is that a good thing or a bad thing (or maybe doesn't matter) for css
variables/custom properties?