[css-position-3] Define positioning scheme terms up front.

Author: fantasai

css-position-3/Overview.bs

@@ -199,7 +199,7 @@ Choosing A Positioning Scheme: 'position' property</h2>
 	Animation type: discrete
 	</pre>
 
-	The 'position' property determines which of the positioning schemes
+	The 'position' property determines which of the <dfn export lt="positioning scheme">positioning schemes</dfn>
 	is used to calculate the position of a box.
 	Values other than ''static'' make the box a <dfn export lt="positioned box|positioned">positioned box</dfn>,
 	and cause it to establish an [=absolute positioning containing block=] for its descendants.
@@ -219,6 +219,8 @@ Choosing A Positioning Scheme: 'position' property</h2>
 		This offsetting is a purely visual effect,
 		and does not affect the size or position of any other box,
 		except insofar as it increases the [=scrollable overflow area=] of its ancestors.
+		This [=positioning scheme=] is called
+		<dfn export dfn lt="relative position|relatively position">relative positioning</dfn>.
 
 	<dt><dfn>sticky</dfn>
 	<dd>
@@ -227,6 +229,8 @@ Choosing A Positioning Scheme: 'position' property</h2>
 		in reference to the nearest ancestor [=scroll container's=] [=scrollport=]
 		as modified by the [=inset properties=],
 		in the axes whose [=inset property=] values are not both ''top/auto''.
+		This [=positioning scheme=] is called
+		<dfn export dfn lt="sticky position">sticky positioning</dfn>.
 
 	<dt><dfn>absolute</dfn>
 	<dd>
@@ -239,6 +243,8 @@ Choosing A Positioning Scheme: 'position' property</h2>
 		as modified by the box's [=inset properties=].
 		It can overlap in-flow siblings
 		or other [=absolutely positioned=] elements.
+		This [=positioning scheme=] is called
+		<dfn export dfn lt="absolute position|absolutely position">absolute positioning</dfn>.
 
 	<dt><dfn>fixed</dfn>
 	<dd>
@@ -248,6 +254,9 @@ Choosing A Positioning Scheme: 'position' property</h2>
 		The box’s position is fixed with respect to this reference rectangle:
 		it does not move when the document is scrolled,
 		and is replicated on every page when it is paginated.
+		This [=positioning scheme=] is called
+		<dfn export dfn lt="fixed position">fixed positioning</dfn>
+		and is considered a subset of [=absolute positioning=].
 
 		<div class="example">
 			Authors may wish to specify ''fixed'' in a media-dependent way.
@@ -488,14 +497,13 @@ Box Insets Shorthands: the 'inset-block', 'inset-inline', and 'inset' properties
 	exactly as the 'margin' property does for its longhands.
 
 
-
 <h3 id="rel-pos">
 Relative positioning</h3>
 
 	Once a box has been laid out according to the <a>normal flow</a> or <a>floated</a>,
 	it may be offset relative to this position.
 
-	This is called <dfn export lt="relative position|relatively position|relpos">relative positioning</dfn>.
+	This is called [=relative positioning=]
 	Offsetting a box (B1) in this way has no effect on the box (B2) that follows:
 	B2 is given a position as if B1 were not offset
 	and B2 is not re-positioned after B1’s offset is applied.
@@ -598,7 +606,7 @@ Effects of Relative Positioning On Table Elements</h4>
 <h3 id="sticky-pos">
 Sticky positioning</h3>
 
-	A <dfn export lt="sticky position|sticky position|stickypos">sticky positioned</dfn> box
+	A [=sticky positioned=] box
 	is positioned similarly to a <a>relatively positioned</a> box,
 	but the offset is computed with reference to the nearest ancestor with a <a>scrolling box</a>,
 	or the viewport if no ancestor has a <a>scrolling box</a>.