[css-sizing-3] Use <<length>> rather than definite to avoid complexity around percentages etc. w3c#4217

Author: fantasai

css-sizing-3/Overview.bs

@@ -835,9 +835,10 @@ Intrinsic Sizes</h3>
 				use the <a>stretch fit</a> into that size for the inline size
 				and calculate the block size using the aspect ratio.
 			* Otherwise
-				if the box has a [=definite=] non-''min-width/auto'' value for 'min-width' or 'min-height',
+				if the box has a <<length>> as its [=computed value=]
+				for 'min-width' or 'min-height',
 				use that size and calculate the other dimension using the aspect ratio;
-				if both dimensions have a definite minimum,
+				if both dimensions have a <<length>> minimum,
 				choose the one that results in the larger overall size.
 
 				Note: This case was previous calculated from a 300x150 default size,
@@ -856,7 +857,8 @@ Intrinsic Sizes</h3>
 				in the appropriate dimension,
 				use the <a>stretch fit</a> into that size in that dimension.
 			* Otherwise,
-				if the box has a <a>definite</a> non-''min-width/auto'' minimum size ('min-width'/'min-height') in that dimension,
+				if the box has a <<length>> as its [=computed|computed value=]
+				[=minimum size=] ('min-width'/'min-height') in that dimension,
 				use that size.
 			* Otherwise,
 				use ''300px'' for the width