[selectors-4] Whoops I wrote an essay about the ::-webkit- quirk.

Author: tabatkins

selectors-4/Overview.bs

@@ -3808,6 +3808,59 @@ Appendix B: Obsolete but Required Parsing Quirks for Web Compat</h2>
 		These otherwise-unsupported pseudo-elements
 		must be serialized in ASCII lowercase.
 
+		<details class=note>
+			<summary>What's this quirk about?</summary>
+
+			Selectors have long had a behavior
+			where a single unknown/invalid selector
+			invalidates the entire selector list
+			(rather than just invaliding the one complex selector it finds itself in).
+			This is generally considered a legacy mistake by the WG,
+			but can't be fixed at this point,
+			as too many stylesheets depend on this behavior,
+			intentionally or not.
+
+			One aspect of this is that any use of vendor-specific selectors,
+			which are generally not recognized by other User Agents,
+			invalidate the entire selector,
+			and take the entire style rule down with them.
+			This has been used intentionally in the past--
+			in the severely not-recommended practice of hiding style rules from some browsers
+			by making them invalid in every other browser--
+			and unintentionally,
+			with people styling an element
+			and also applying those styles to a vendor-specific pseudo-element
+			(such as the various <{input}>-related pseudos some browsers expose),
+			not realizing that this hides the entire rule from other browsers.
+
+			In addition to this more general reasoning,
+			WebKit-derived user agents,
+			such as Safari or Chrome,
+			have an additional quirk related to their vendor-prefixed pseudo-elements,
+			where any <css>::-webkit-</css>-prefixed selectors is considered valid at parse time.
+			(This is probably a leftover quirk of an early CSS feature,
+			since dropped,
+			that intentionally treated all possible pseudo-elements as valid at parse time,
+			in anticipation of a feature letting authors define their own pseudo-elements.)
+
+			Similar to other legacy quirks,
+			such as those documented in [[QUIRKS]],
+			this particular vendor-specific oddity has become common enough
+			that other user agents are seeing sites breaking due to them depending on it,
+			accidentally or not.
+			As such, since the quirk is thus,
+			in practical terms,
+			required to render the modern web correctly,
+			specifying it and requiring it for all user agents
+			ensures that today's web pages are more likely to be correctly rendered
+			in user agents both current and future.
+
+			As usual with quirks, however,
+			webpages intentionally relying on this
+			will be met with shaming and derision from members of the CSSWG,
+			and all right-thinking web developers.
+		</details>
+
 
 <h2 id="changes">
 Changes</h2>