[css-text-4] Add note about deviations from UAX-14 (w3c#9997)

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

Author: birtles

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.