[css-pseudo-4] Rewrite definition of first formatted line as a proper definition as outlined by Oriol in #1158

Author: fantasai

css-pseudo-4/Overview.bs

@@ -117,40 +117,51 @@ The ::first-line pseudo-element</h3>
 Finding the First Formatted Line</h4>
 
   In CSS, the ''::first-line'' pseudo-element
-  can only have an effect when attached to a <a>block container</a>.
-  The <dfn export>first formatted line</dfn> of an element
-  must occur inside a block-level descendant in the same flow
-  (i.e., a block-level descendant that is not out-of-flow due to floating or positioning).
-
-  <div class="example">
-    For example, the first line of the <code>DIV</code> in <code>&lt;DIV&gt;&lt;P&gt;This line...&lt;/P&gt;&lt;/DIV&gt;</code>
-    is the first line of the <code>P</code>
-    (assuming that both <code>P</code> and <code>DIV</code> are blocks).
-  </div>
-
-  The first line of a [=block container=] which does not participate in a [=block formatting context=]
-  (and likewise the first line of a [=table caption=])
+  can only have an effect when attached to a <a>block container</a>:
+
+  * The <dfn export>first formatted line</dfn> of
+    a [=block container=] that establishes an [=inline formatting context=]
+    contains the [=inline-level=] content of its first [=line box=].
+  * The [=first formatted line=] of
+    a [=block container=] or [=multi-column container=]
+    that contains [=block-level=] content
+    (and is not a [=table wrapper box=])
+    is the [=first formatted line=] of
+    its first [=in-flow=] [=block-level=] child.
+    If no such line exists,
+    it has no [=first formatted line=].
+
+  Note: The [=first formatted line=] can be an empty line.
+  For example, the first line of the <code>p</code> in
+  <code>&lt;p&gt;&lt;br&gt;First&hellip;</code>
+  doesn't contain any letters.
+  Thus the word “First” is not on the first formatted line,
+  and will not be affected by ''p::first-line''.
+
+  Note: The first line of a [=block container=]
+  that does not participate in a [=block formatting context=]
   cannot be the first formatted line of an ancestor element.
   Thus, in <code>&lt;DIV&gt;&lt;P STYLE="display: inline-block"&gt;Hello&lt;BR&gt;Goodbye&lt;/P&gt; etcetera&lt;/DIV&gt;</code>
-  the first formatted line of the <code>DIV</code> is not the line "Hello".
+  the first formatted line of the <code>DIV</code> is not the line “Hello”,
+  but rather the (otherwise empty) line that contains that entire inline block.
 
-  Note: Note that the first line of the <code>p</code> in this fragment:
-  <code>&lt;p&gt;&lt;br&gt;First...</code>
-  doesn't contain any letters (assuming the default style for <code>br</code>).
-  The word "First" is not on the first formatted line.
-
-  A user agent must act as if the fictional start tags of a ''::first-line'' pseudo-element
-  were nested just inside the innermost enclosing block-level element.
+  When a [=first formatted line=] is represented
+  by multiple ''::first-line'' pseudo-elements,
+  they are nested in the same order as their [=originating elements=].
 
   <div class="example">
-    For example, the <a>fictional tag sequence</a> for
+    Consider the following markup:
     <pre>
       &lt;DIV&gt;
         &lt;P&gt;First paragraph&lt;/P&gt;
         &lt;P&gt;Second paragraph&lt;/P&gt;
       &lt;/DIV&gt;
     </pre>
-    is
+
+    If we assume a [=fictional tag sequence=] to represent
+    the elements’ ''::first-line'' pseudo elements,
+    it would be something like:
+
     <pre>
       &lt;DIV&gt;
         &lt;P&gt;&lt;DIV::first-line&gt;&lt;P::first-line&gt;First paragraph&lt;/P::first-line&gt;&lt;/DIV::first-line&gt;&lt;/P&gt;