[css-syntax-3] Per WG resolution, {} blocks are only allowed as the entire value of a property. #9317

Author: tabatkins

css-syntax-3/Overview.bs

@@ -2828,6 +2828,79 @@ Consume a block's contents</h4>
 			append it to |rules|.
 	</dl>
 
+	<details class=note>
+		<summary>Implementation note</summary>
+
+		This spec, as with many CSS specs,
+		has been written to prioritize understandability
+		over efficiency.
+		A number of algorithms,
+		notably the above "parse as a declaration, then parse as a rule" behavior
+		can be fairly inefficient
+		if implemented naively as described.
+
+		However, the behavior has been carefully written
+		to allow "early exits" as much as possible.
+		In particular,
+		and roughly in order of when the exit can occur:
+
+		* If the first non-whitespace token
+			isn't an <<ident-token>>
+			for a recognized property name (or a custom property name),
+			you can immediately stop parsing as a declaration
+			and reparse as a rule instead.
+			If the <em>next</em> non-whitespace token isn't a <<colon-token>>,
+			you can similarly immediately stop parsing as a declaration.
+
+			(That is, ''font+ ...'' is guaranteed to not be a property,
+			nor is <css>not-a-prop-name: ...</css>.)
+
+		* If the first two non-whitespace tokens
+			are a custom property name and a colon,
+			it's definitely a custom property
+			and won't ever produce a valid rule,
+			so even if the custom property ends up invalid
+			there's no need to try and reparse as a rule.
+
+			(That is, ''--foo:hover {...}'' is guaranteed to be a custom property,
+			not a rule.)
+
+		* If the first three non-whitespace tokens
+			are a valid property name, a colon, and anything other than a <<{-token>>,
+			and then while parsing the declaration's value you encounter a <<{-token>>,
+			you can immediately stop parsing as a declaration
+			and reparse as a rule instead.
+
+			(That is, ''font:bar {...'' is guaranteed to be an invalid property.)
+
+		* If you see a recognized property name, a colon, and a {}-block,
+			but the first non-whitespace tokens following that block
+			isn't either immediately the final semicolon,
+			or the !important followed by the semicolon,
+			you can immediately stop parsing as a declaration
+			and reparse as a rule instead.
+
+			(That is, ''font: {} bar ...'' is guaranteed to be an invalid property;
+			you don't need to keep parsing until you hit a semicolon.)
+
+		Similarly,
+		even tho the parsing requirements are specified
+		to rely on checking the grammar of the declarations
+		as you parse,
+		a <em>generic</em> processor
+		trying to implement a non-CSS language
+		on top of the generic CSS <em>syntax</em>
+		can still get away with just verifying that declarations
+		start with an ident, a colon,
+		and then either contain solely a {}-block
+		or no {}-block at all.
+		They'll just spent a little more time on parsing
+		than an implementation with grammar knowledge
+		in cases like ''foo:hover ... {}'',
+		since they can't early-exit on the first token.
+	</details>
+
+
 
 <h4 id="consume-declaration">
 Consume a declaration</h4>
@@ -2873,19 +2946,6 @@ Consume a declaration</h4>
 			and with <<semicolon-token>> as the stop token,
 			and set |decl|'s value to the result.
 
-			If |decl|'s name is a [=custom property name string=],
-			then set |decl|'s |original text|
-			to the segment of the original source text string
-			corresponding to the tokens
-			returned by the [=consume a list of component values=] call.
-
-			If |decl|'s name is an [=ASCII case-insensitive=] match for "unicode-range",
-			[=consume the value of a unicode-range descriptor=]
-			from the segment of the original source text string
-			corresponding to the tokens
-			returned by the [=consume a list of component values=] call,
-			and replace |decl|'s value with the result.
-
 		<li>
 			If the last two non-<<whitespace-token>>s in |decl|'s value are
 			a <<delim-token>> with the value "!"
@@ -2897,6 +2957,27 @@ Consume a declaration</h4>
 			While the last item in |decl|'s value is a <<whitespace-token>>,
 			[=list/remove=] that token.
 
+		<li>
+			If |decl|'s name is a [=custom property name string=],
+			then set |decl|'s |original text|
+			to the segment of the original source text string
+			corresponding to the tokens
+			of |decl|'s value.
+
+			Otherwise, if |decl|'s value contains a top-level [=simple block=]
+			with an associated token of <<{-token>>,
+			and also contains <em>any other</em> non-<<whitespace-token>> value,
+			return nothing.
+			(That is, a top-level {}-block is only allowed
+			as the entire value of a non-custom property.)
+
+			Otherwise, if |decl|'s name is an [=ASCII case-insensitive=] match for "unicode-range",
+			[=consume the value of a unicode-range descriptor=]
+			from the segment of the original source text string
+			corresponding to the tokens
+			returned by the [=consume a list of component values=] call,
+			and replace |decl|'s value with the result.
+
 		<li>
 			If |decl| is valid in the current context,
 			return it;