[css-inline] Add section about shaping. Current text based on known examples thus far, may change with more information.

Author: fantasai

css-inline/Overview.bs

@@ -291,6 +291,16 @@ Font size of drop cap = ((N-1) * line-height + [cap-height of para] * [font size
 
 The line height used in this calculation is the 'line-height' of the containing block (or, in the case where a baseline grid is in use, the baseline-to-baseline spacing required by the baseline grid [[CSS3-LINE-GRID]]). The contents of the lines spanned, and therefore any variation in their heights and positions, is not accounted for.
 
+<h4 id="initial-letter-shaping">Shaping and Glyph Selection</h4>
+
+When 'initial-letter' is not ''normal'', shaping does not occur across the box's boundaries,
+as if the zero-width non-joiner (U+200C) were inserted before/after the <a>initial letter</a>.
+For example, if the first letter of the Farsi word “پس” were styled with ''initial-letter: 2 1'',
+both letters would be styled in their isolated forms,
+with “پ” as the initial letter, followed by the normally-styled “س”.
+
+Issue: Are there other things we need to consider here?
+
 <h3 id="initial-letter-exclusions">Space Around Initial Letters</h3>
 
 The glyph(s) of an initial letter do not always fit within the specified sink. For example, if an initial letter has a descender, it could crash into the (n+1)th line of text. This is not desirable.