[css-rhythm-1] Improve intro to block-step properties. #1154

Author: fantasai

css-rhythm-1/Overview.bs

@@ -110,13 +110,15 @@ or that the 'line-height-step' property is used together with them.
 Adjusting Block-level Box Heights {#block-height}
 =================================
 
-	Much web content includes blocks with varying font sizes, line heights,
-	margins, etcetera. Adjusting line box heights may not be enough to
-	achieve a vertical rhythm in these circumstances,
-	and can negatively affect the readability of Latin scripts by altering line heights.
-	Constraining block height, rather than line height, to a multiple of a step size
-	can achieve a vertical rhythm in these circumstances.
-
+	The most common typographic rhythm problems are block-level intrusions:
+	when continuous paragraph-level text is interrupted
+	by differently-sized content such as illustrations and headings,
+	then the line-to-line rhythm can be thrown off.
+	The block step properties allow such elements
+	to be fitted to the rhythmic interval
+	by constraining their height to a multiple of a specified step size.
+	This allows content before and after the interruption
+	to maintain a continuous rhythm.
 
 		<div class="figure">
 			<img src="images/block-step-size-before.png"
@@ -134,6 +136,21 @@ Adjusting Block-level Box Heights {#block-height}
 			restores vertical rhythm.</figcaption>
 		</div>
 
+	While consistent use of the block step properties
+	can produce the strictly gridded layout needed
+	for parallel content flows,
+	per-element specification of the step size
+	also allows some interruptions to take their natural size in the flow,
+	restarting the vertical rhythm afterwards.
+	This can be useful in single-column layouts
+	where the vertical rhythm is important to maintain visual continuity,
+	but there is nothing alongside to align to.
+	In these casese,
+	large interruptions which visually disconnect the flow before and after
+	can prioritize optimal spacing around the item
+	over strict adherence to a continouous grid
+	(by specifying ''block-step: none'', the initial value).
+
 	ISSUE: This proposal can be simplified down to just the 'block-step-size' property, represented solely through its shortened form as 'block-step'.
 	This level will likely at most contain 'block-step-size' and 'block-step-insert', leaving 'block-step-align' and 'block-step-round' to be added if the future demands.
 	The full design is described herein for current discussion and future reference.