fix: allow double hyphen css selector names (#15384)

.changeset/fast-pants-decide.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: allow double hyphen css selector names

packages/svelte/src/compiler/phases/1-parse/read/style.js

@@ -566,7 +566,7 @@ function read_attribute_value(parser) {
 }
 
 /**
- * https://www.w3.org/TR/CSS21/syndata.html#value-def-identifier
+ * https://www.w3.org/TR/css-syntax-3/#ident-token-diagram
  * @param {Parser} parser
  */
 function read_identifier(parser) {
@@ -574,7 +574,7 @@ function read_identifier(parser) {
 
 	let identifier = '';
 
-	if (parser.match('--') || parser.match_regex(REGEX_LEADING_HYPHEN_OR_DIGIT)) {
+	if (parser.match_regex(REGEX_LEADING_HYPHEN_OR_DIGIT)) {
 		e.css_expected_identifier(start);
 	}
 

packages/svelte/tests/css/samples/double-hyphen/expected.css

@@ -0,0 +1,4 @@
+
+	.--foo.svelte-xyz {
+		color: red;
+	}

packages/svelte/tests/css/samples/double-hyphen/input.svelte

@@ -0,0 +1,7 @@
+<div class="--foo"></div>
+
+<style>
+	.--foo {
+		color: red;
+	}
+</style>