Merge pull request w3c#470 from cdoublev/issue-7727

Author: tabatkins

css-masking-1/Overview.bs

@@ -1015,7 +1015,7 @@ The 'mask-mode' and 'mask-type' properties must have no affect on the <a>mask bo
 
 <pre class=propdef>
 Name: mask-border-slice
-Value: <<number-percentage>>{1,4} fill?
+Value: [ <<number>> | <<percentage>> ]{1,4} fill?
 Initial: 0
 Applies to: All elements. In SVG, it applies to <a>container elements</a> excluding the <a element>defs</a> element, all <a>graphics elements</a> and the <a element>use</a> element
 Inherited: no