[css-containment] Clarify ambiguous phrasing (#831)

In a few places, this is an editorial change merely making sure
that there is no way to misinterpret what was meant.
In addition this gives a normative definition
to the previously vague requirement that
in certain cases an element "must be a formatting context".

Closes #830

Author: frivoal

css-containment/Overview.bs

@@ -184,10 +184,7 @@ Layout Containment</h3>
 
 	Giving an element <dfn export>layout containment</dfn> has the following effects:
 
-	1. The element must be a <a>formatting context</a>.
-
-		Issue: This needs to specify how it becomes a formatting
-		context (and which type it becomes).
+	1. The element <a>becomes a formatting context</a>.
 
 	2. If a <a>fragmentation context</a> participates in <a>layout containment</a>,
 		the first element with <a>layout containment</a>
@@ -286,9 +283,9 @@ Paint Containment</h3>
 		to access or indicate the presence of the clipped content;
 		nor does it inhibit the creation of any such mechanism through other properties,
 		such as 'overflow', 'resize', or 'text-overflow'.
-	2. The element must act as a containing block for absolutely positioned and fixed positioned descendants.
-	3. The element must be a <a>stacking context</a>.
-	4. The element must be a <a>formatting context</a>.
+	2. The element acts as a containing block for absolutely positioned and fixed positioned descendants.
+	3. The element creates a <a>stacking context</a>.
+	4. The element <a>becomes a formatting context</a>.
 
 	Possible optimizations that can be enabled by <a>paint containment</a> include (but are not limited to):
 
@@ -308,6 +305,15 @@ Paint Containment</h3>
 	3. Because they are guaranteed to be stacking contexts,
 		scrolling elements can be painted into a single GPU layer.
 
+<h2 id="becoming-formatting-context">Becoming a formatting context</h2>
+
+When required by <a>layout containment</a> and <a>paint containment</a>,
+an element may need to <dfn lt='become a formatting context|becomes a formatting context'>become a <a>formatting context</a></dfn>.
+If the element already establishes a <a>formatting context</a> of any kind,
+this condition is satisfied.
+Otherwise, it is made to establish a <a>BFC</a> by changing its <a>inner display type</a> to ''flow-root''.
+
+
 Privacy and Security Considerations {#privsec}
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