[css-grid] Fix some grid-gap minor errors.

Author: tabatkins

css-grid/Overview.bs

@@ -2934,31 +2934,33 @@ Gutters: the 'grid-column-gap', 'grid-row-gap', and 'grid-gap' properties</h3>
 	Inherited: no
 	Percentages: n/a
 	Media: visual
-	Computed value: specified value
+	Computed value: absolute <<length>>
+	Animatable: as <<length>>
 	</pre>
 
 	These properties specify the <dfn lt=gutter>gutters</dfn> between
 	<a>grid rows</a> and <a>grid columns</a>, respectively.
 	The effect is as though the affected <a>grid lines</a> acquired width:
 	the <a>grid track</a> between two <a>grid lines</a>
-	is the space between the <i>gutters</i> that represent them.
+	is the space between the <a>gutters</a> that represent them.
 	For the purpose of <a href="#algo-track-sizing">track sizing</a>,
-	each <i>gutter</i> is essentially treated as an extra track of the specified length.
+	each <a>gutter</a> is essentially treated as an extra track of the specified length.
+	Negative values are invalid.
 
 	Note: Additional spacing may be added between tracks
 	due to 'justify-content'/'align-content'.
 	See [[#algo-overview]].
-	This space effectively increases the size of the <i>gutters</i>.
+	This space effectively increases the size of the <a>gutters</a>.
 
 	<pre class='propdef'>
 	Name: grid-gap
 	Value: <<'grid-column-gap'>> <<'grid-row-gap'>>?
-	Initial: normal
-	Applies to: <a>grid containers</a>
-	Inherited: no
-	Percentages: n/a
-	Media: visual
-	Computed value: specified value
+	Initial: see individual properties
+	Applies to: see individual properties
+	Inherited: see individual properties
+	Percentages: see individual properties
+	Media: see individual properties
+	Computed value: see individual properties
 	</pre>
 
 	This property is a <a>shorthand</a> that sets 'grid-column-gap' and 'grid-row-gap' in one declaration.