[css-pseudo-4] Use minus as difference operator. #5830

Backslash might be nice alternative, but turns into a yen sign in some fonts. :/

Author: fantasai

css-pseudo-4/Overview.bs

@@ -339,7 +339,7 @@ First-Letter Text: ''::first-letter'' pseudo-element and its ''::prefix'' and ''
     (ignoring the exclusion of word separators from <code>Zs</code>)
     represented as
     <code highlight="bnf" style="font-family: inherit">
-    (P (Zs|P)*)? (L|N|S)+ ((Zs|P^(Ps|Pd))* (P^(Ps|Pd))?</code>
+    (P (Zs|P)*)? (L|N|S)+ ((Zs|P−(Ps|Pd))* (P−(Ps|Pd))?</code>
     or, alternatively,
     <code highlight="bnf" style="font-family: inherit">
     ([P] [Zs P]*)? [L N S]+ ([[Zs] [P--[Ps Pd]]]* [P--[Ps Pd]])?</code>