[css-text-4] Add note about deviations from UAX-14 (#9997)

Co-authored-by: Florian Rivoal <git@florian.rivoal.net>

css-text-4/Overview.bs

@@ -7462,6 +7462,34 @@ Line Breaking Strictness: the 'line-break' property</h3>
  (ตัวอย่าง·การ·เขียน·ภาษา·ไทย).
  </div>
 
+ <div class=note>
+ Note: While user agents can refer to [[!UAX14]] as a starting point
+ for their line-breaking implementation,
+ the following deviations could be desirable
+ for maximum interoperability with existing implementations:
+
+ - Not introducing a line break opportunity between
+ U+0021 (Exclamation Mark, <code>!</code>) and
+ a letter ([=Unicode general category=] <code>L</code>).
+ This prevents a break in the string “!important”.
+ - Not introducing a line break opportunity between
+ U+002F (Solidus, <code>/</code>) and
+ a letter ([=Unicode general category=] <code>L</code>).
+ This prevents a break in dates such as “23/Jan/2024”.
+ - Not introducing a line break opportunity between
+ U+007C (Vertical Line, <code>|</code>) and
+ a letter ([=Unicode general category=] <code>L</code>).
+ - Not introducing a line break opportunity between
+ U+002D (Hyphen-Minus `-`) and
+ a digit ([=Unicode general category=] <code>Nd</code>)
+ if the codepoint prior to the hyphen was _not_ a letter or digit
+ ([=Unicode general category=] <code>L</code> or <code>Nd</code>).
+ This prevents breaking after the minus sign before a number
+ such as in “-13”
+ whilst allowing breaks after the hyphen in “ABCD-1234” and “1234-5678”
+ which may appear in long URLs, for example.
+ </div>
+
  Note: The CSSWG recognizes that in a future edition of the specification
  finer control over line breaking may be necessary
  to satisfy high-end publishing requirements.