[css-transforms] Link to dfn of local/viewport coordinate system

Author: xfq

css-transforms/Overview.bs

@@ -131,7 +131,7 @@ When used in this specification, terms have the meanings assigned in this sectio
 :: A matrix that defines the mathematical mapping from one coordinate system into another. It is computed from the values of the 'transform' and 'transform-origin' properties as described <a href="#transformation-matrix-computation">below</a>.
 
 : <dfn>current transformation matrix</dfn> (CTM)
-:: A matrix that defines the mapping from the local coordinate system into the viewport coordinate system.
+:: A matrix that defines the mapping from the [=local coordinate system=] into the [=viewport coordinate system=].
 
 : <dfn>2D matrix</dfn>
 ::  A 3x2 transformation matrix with 6 items or a 4x4 matrix with 16 items, where the items m<sub>31</sub>, m<sub>32</sub>, m<sub>13</sub>, m<sub>23</sub>, m<sub>43</sub>, m<sub>14</sub>, m<sub>24</sub>, m<sub>34</sub> are equal to ''0'' and m<sub>33</sub>, m<sub>44</sub> are equal to ''1''.