[css-text-decor-4] Add back in detailed description of line position and thickness from old drafts of Level 3. (This may need to be tweaked, this is just a direct copy.)

Author: fantasai

css-text-decor-4/Overview.bs

@@ -25,8 +25,6 @@ It has no status at W3C and has not yet been discussed by the CSSWG.
 
 	<ul>
 		<li>text-shadow with spread
-		<li>a text-decoration-offset property
-		<li>a text-line-width property, see <a href="https://www.w3.org/mid/9500A3A5-957D-462D-B30C-E5C548ADEB0E@verou.me">thread</a>
 	</ul>
 
 <h3 id="text-decoration-width-property">
@@ -68,7 +66,98 @@ Automatic Thickness of Text Decoration Lines</h4>
 
 	Issue: Insert L3 text here.
 
+<h3 id="line-position">
+Determining the Position and Thickness of Line Decorations</h3>
+
+	Issue: This section is copied over from early drafts of Text Decoration Level 3, and is still under discussion.
+
+	<p>Since line decorations can span elements with varying font sizes and
+		vertical alignments, the best position for a line decoration is not
+		necessarily the ideal position dictated by the <i>decorating box</i>.
+		Instead, it's calculated, per line, from all text decorated by the <i>decorating box</i> on that line,
+		the <dfn>considered text</dfn>.
+		However, descendants of the <i>decorating box</i>
+		that are skipped due to 'text-decoration-skip',
+		descendant inlines with ''text-decoration-skip: ink'',
+		and any descendants that do not participate in the <i>decorating box</i>’s inline formatting context
+		are excluded from the set of <i>considered text</i>.
+
+	<p>
+		The line decoration positions are then calculated
+		per line
+		as follows
+		(treating <a href="#underline-left"><i>over</i>-positioned underlines</a> as <i>over</i> lines
+		and <a href="#underline-left"><i>under</i>-positioned overlines</a> as <i>under</i> lines):
+
+	<dl>
+		<dt><a href="https://www.w3.org/TR/css-writing-modes/#over"><i>over</i></a> lines
+		<dd>
+			Align the line decoration with respect to the highest
+			<a href="https://www.w3.org/TR/css-writing-modes/#over"><i>over</i></a> EM-box edge
+			of the <i>considered text</i>.
+
+		<dt><a href="#underline-alphabetic"><i>alphabetic</i></a> underlines
+		<dd>
+			<p>The alphabetic underline position is calculated by taking
+			the ideal offset (from the alphabetic baseline) of each run of <i>considered text</i>,
+			averaging those, and then using the lowest alphabetic baseline to actually position the line.
+			(Alphabetic baselines can differ between ''baseline''-aligned boxes
+			 if the dominant baseline is non-alphabetic.)
+			To prevent superscripts and subscripts from throwing this position off-kilter,
+			an inline with a non-initial computed 'vertical-align'
+			is treated as having the ideal underline position of its parent.
+
+		<dt>non-alphabetic <a href="https://www.w3.org/TR/css-writing-modes/#under"><i>under</i></a> lines
+		<dd>
+			Position the line decoration with respect to the lowest
+			<a href="https://www.w3.org/TR/css-writing-modes/#under"><i>under</i></a> EM-box edge
+			of the <i>considered text</i>.
+
+		<dt>line-throughs
+		<dd>
+			Line-throughs essentially use the same sort of averaging as for alphabetic underlines,
+			but recompute the position when drawing across a descendant with a different computed 'font-size'.
+			(This ensures that the text remains effectively “crossed out” despite any font size changes.)
+			For each run of <i>considered text</i> with the same 'font-size',
+			compute an ideal position averaged from its font metrics.
+			To prevent superscripts and subscripts from throwing this position off-kilter,
+			an inline with a non-initial computed 'vertical-align'
+			is treated as having the ideal underline position of its parent.
+			Position the portion of the line across each decorated fragment at that position.
+
+			<p class='issue'>
+				For simplicity, line-throughs should draw over each element at that element's preferred/averaged position.
+				This can produce some undesirable jumpiness,
+				but there doesn't appear to be any way to avoid that which is correct in all instances,
+				and all attempts are worryingly complex.
+				What position should line-throughts adopt over elements that have a different font-size,
+				but no <i>considered text</i>?
+	</dl>
 
+	<p>
+		CSS does not define the thickness of line decorations.
+		In determining the thickness of text decoration lines,
+		user agents may consider the font sizes, faces, and weights of descendants
+		to provide an appropriately averaged thickness.
+
+	<div class="example">
+		<p>The following figure shows the averaging for underline:
+		<p><img alt="In the first rendering of the underlined text '1st a'
+								 with 'st' as a superscript, both the '1st' and the 'a'
+								 are rendered in a small font. In the second rendering,
+								 the 'a' is rendered in a larger font. In the third, both
+								 '1st' and 'a' are large."
+						height="105" src="underline-averaging.gif" width="326">
+		<p>In the three fragments of underlined text, the underline is drawn
+			consecutively lower and thicker as the ratio of large text to small
+			text increases.</p>
+		<p>Using the same example, a line-through would in the second fragment,
+		instead of averaging the two font sizes,
+		split the line-through into two segments:
+		<p><img alt="" src="linethrough-averaging.gif">
+		<p>In both cases, however, the superscript, due to the vertical-alignment shift,
+		has no effect on the position of the line.
+	</div>
 
 <h3 id="underline-offset">
 Text Underline Offset: the 'text-underline-offset' property</h3>