[css-transforms-2] Add a definition for the 2D transform functions to fix HTML validation error.

Author: dbaron

css-transforms-2/Overview.bs

@@ -971,10 +971,9 @@ For example, ''scale3d(50%, 100%, 150%)'' serializes as ''scale3d(0.5, 1, 1.5)''
 The scale functions defined in [[!css-transforms-1]] now support percentages.
 
 : <span class='prod'><dfn>scale()</dfn> = scale( [ <<number>> | <<percentage>> ]#{1,2} )</span>
-
 : <span class='prod'><dfn>scaleX()</dfn> = scaleX( [ <<number>> | <<percentage>> ] )</span>
-
 : <span class='prod'><dfn>scaleY()</dfn> = scaleY( [ <<number>> | <<percentage>> ] )</span>
+:: As <a href="https://www.w3.org/TR/css-transforms-1/#two-d-transform-functions">defined in css-transforms-1</a>, but also accepting percentages as <a href="#transform-functions">described above</a>.
 
 3D Transform Functions {#three-d-transform-functions}
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