[css-round-display] Add detail description of Polar

Author: Jihye Hong

css-round-display/Overview.bs

@@ -273,10 +273,10 @@ The example below shows how the 'border-boundary' property works on drawing bord
 <h2 id="positioning-content">Positioning content by using the polar coordinate system</h2>
 
 <p>
-Elements could be often placed along a circle or concentric circles, and the polar coordinate system is useful to handle such cases. This section introduces polar positioning to support layout of elements in the polar coordinate system where the position of an element is determined by a distance from the center point within the containing element and an angle from the Y-axis. We add '<code>polar</code>' to the value of the 'position' property to support the polar coordinates of an element itself. The 'polar-angle' and 'polar-distance' properties specify the position of an element.
+Polar coordinate system is a two-dimensional coordinate system that describes the position of a point in a plane with a distance from a reference point and an angle from a reference direction. Elements could be placed along a circle or concentric circles, and the polar coordinate system is useful to handle such cases. This section introduces polar positioning to support layout of elements in the polar coordinate system where the position of an element is determined by a distance from the center point within the containing element and an angle from the Y-axis. We add '<code>polar</code>' to the value of the 'position' property to support the polar coordinates of an element itself. The 'polar-angle' and 'polar-distance' properties specify the position of an element.
 </p>
 <p class="note">
-Note: In the polar coordinate system, a point is described as being a certain distance from the pole and a certain angle from the polar axis. In the mathematical theory, the polar axis is commonly defined as the positive direction of the x-axis, but we consider the polar axis as the positive direction of the y-axis position as other CSS specifications usually do.
+Note: In the polar coordinate system, a point is described as being a certain distance from the pole and a certain angle from the polar axis. In the mathematical theory, the polar axis is commonly defined as the positive direction of the x-axis, but we consider the polar axis as the positive direction of the y-axis position as other CSS specifications usually do. Therefore, when the 'polar-angle'  value  of an element is 0, the element is positioned on the y-axis. If the angle value of an element increases in the positive direction from 0, the element moves clockwise. The method to determine a direction using 'polar-angle' works the same way in [[CSS-VALUES]], <<angle>> value.
 </p>
 <h3 id="position-property">The 'position' property</h3>
 When the 'position' property on an element is set to '<code>polar</code>', the element could be affected in polar coordinate system.
@@ -291,6 +291,9 @@ When the 'position' property on an element is set to '<code>polar</code>', the e
 	Computed value: specified value
 	Animatable: no
 </pre>
+<p class="note">
+Note:  The center point of the containing block of an element indicates the origin of polar coordinate when the value of position property of the element  is '<code>polar</code>'.
+</p>
 <h3 id="polar-angle-property">The 'polar-angle' property</h3>
 The 'polar-angle' property specifies the angle from the Y-axis. This property is activated on condition of position: polar.
 <pre class='propdef'>