[css-overflow-4] continue:discard makes the box establish an IFC

See #10323

css-overflow-4/Overview.bs

@@ -975,45 +975,6 @@ Forcing a Break After a Set Number of Lines: the 'max-lines' property</h3>
  If fewer than <var>N</var> line boxes exist,
  then 'max-lines' introduces no <a>region break</a>.
 
- <div class=example>
- As ''continue: discard'' does not cause the element to [=establish an independent formatting context=],
- lines in a nested element with 'line-clamp' are counted,
- as can be seen in the examples below.
- <xmp highlight=markup>
- <div id=a>
- a: line 1<br>
- a: line 2<br>
- <div id=b>
- b: line 1<br>
- b: line 2<br>
- b: line 3<br>
- b: line 4<br>
- </div>
- a: line 3<br>
- a: line 4<br>
- </div>
- </xmp>
- Sample rendering given <code highlight=css>#a { line-clamp: 5; } #b { line-clamp: 2; }</code>:
- <pre>
- a: line 1
- a: line 2
- b: line 1
- b: line 2…
- a: line 3…
- </pre>
-
- Sample rendering given <code highlight=css>#a { line-clamp: 3; } #b { line-clamp: 2; }</code>:
- <pre>
- a: line 1
- a: line 2
- b: line 1…
- </pre>
- Note that in the second case,
- the maximum of 2 lines set on element ''#b'' does not take effect,
- since a forced break is introduced
- before the second line of this element.
- </div>
-
  Note: This implies that 'max-lines' has no effect when applied to [=multi-column containers=],
  since any line box they contain are nested into [=independent formatting contexts=].
 
@@ -1075,15 +1036,15 @@ Fragmentation of Overflow: the 'continue' property</h3>
 
  <dt><dfn>discard</dfn>
  <dd>
- The box becomes a [=fragmentation container=]
+ The box must [=establish an independent formatting context=]
+ and becomes a [=fragmentation container=]
  that captures <a>region breaks</a>,
  if it is not already.
  [[!CSS-BREAK-3]]
  Content after the first [=region break=] is not rendered (see below).
  (If the box is a [=multi-column container=],
  any [=overflow columns=] are also not rendered.)
 
-
  Note: This [=region break=] might be <a lt="forced break">forced</a>
  (e.g. imposed by 'max-lines' or by another mechanism,
  such as the 'break-before'/'break-after' properties)
@@ -1095,8 +1056,6 @@ Fragmentation of Overflow: the 'continue' property</h3>
  do not cause any content to be discarded.
  </dl>
 
- Note: This property does <em>not</em> cause the box to [=establish an independent formatting context=].
-
  <div class=example>
  Given an article with one excessively long overflowing line,
  and four more lines than can fit in the [=fragmentation direction=] as in the illustration bellow,