[css-text-3] break spaces (#111)

* [css-text] Add 'break-spaces'

Based on resolutions from the 2016 San Francisco and Sydney Face-to-face
meetigns.

* [css-text-4] Remove 'pre-wrap-auto' from Level 4

As resolved at the 2016 Sydney face-to-face meeting.
This 'pre-wrap-auto' had already been removed from Level 3,
but was still in level 4.

* [css-text] Rephrase 'break-spaces' description

This does not change the intended behavior, but the previous
phrasing was ambiguous about some situations, and this should clarify.

* [css-text] Fix typo in markup

Author: frivoal

css-text/Overview.bs

@@ -693,6 +693,9 @@ Languages and Typesetting</h4>
          or visually collapse the character advance widths
          of any overflowing spaces
          such that they don't take up space in the line.
+         However, if 'overflow-wrap' is set to ''break-spaces'',
+         collapsing their advance width is not allowed,
+         as this would prevent the preserved spaces from wrapping.
 
          Note: Hanging the white space rather than collapsing it
          allows users to see the space when selecting or editing text.
@@ -1115,15 +1118,17 @@ Line Breaking Details</h3>
 
   <pre class="propdef">
   Name: overflow-wrap, word-wrap
-  Value: normal | break-word
+  Value: normal | break-word || break-spaces
   Initial: normal
   Inherited: yes
   Canonical order: n/a
   </pre>
 
-    <p>This property specifies whether the UA may arbitrarily break within a word
-      to prevent overflow when an otherwise-unbreakable string is too
-      long to fit within the line box. It only has an effect when
+    <p>This property specifies whether the UA may break at otherwise disallowed points within a line
+      to prevent overflow,
+      when an otherwise-unbreakable string is too long to fit within the line box,
+      or when sequences of <a>preserved</a> white space would <a>hang</a>.
+      It only has an effect when
       'white-space' allows <a>wrapping</a>. Possible values:</p>
 
     <dl dfn-for=overflow-wrap dfn-type=value>
@@ -1138,6 +1143,30 @@ Line Breaking Details</h3>
       Shaping characters are still shaped as if the word were not
       broken, and grapheme clusters must stay together as one unit.
       No hyphenation character is inserted at the break point.
+    <dt><dfn>break-spaces</dfn></dt>
+    <dd>Line breaks are handled as usual,
+      except that any sequence of <a>preserved</a> white space
+      that would otherwise overflow the line and <a>hang</a> as per <a href="#white-space-phase-2">Trimming and Positioning</a>
+      must be broken after the last white space character that would fit the line,
+      or after the first white space in the sequence if none would fit,
+      or before the first space in the sequence if none would fit and both ''break-word'' and ''break-spaces'' are specified.
+
+      <p class="note">This last clause is due to the fact that
+      'overflow-wrap: break-words'' allows breaks between any two characters
+      to avoid overflow</p>
+
+      <p class="note">This value does not guarantee that there will never be any overflow due to spaces.
+      For example, if 'overflow-wrap' is set to ''break-spaces''
+      and 'white-space' to ''pre-wrap'',
+      and there is a single <a>preserved</a> space at the end of the line,
+      and that space overflows,
+      the rules of ''break-spaces'' do not introduce any additional line break,
+      and that space does overflow and <a>hang</a>.
+      If 'overflow-wrap' had been set to ''break-word break-spaces'',
+      that space would have been wrapped to the next line,
+      unless the line length is so short that even a single space does not fit,
+      in which case overflow is unavoidable.</p>
+
     </dl>
 
     <p><a>Soft wrap opportunities</a> introduced by ''overflow-wrap: break-word''