release notes for #1665

CHANGELOG.md

@@ -14,7 +14,7 @@
 
 * Disallow certain uses of `<` in `.mts` and `.cts` files
 
-    The upcoming version 4.5 of TypeScript is introducing the `.mts` and `.cts` extensions that turn into the `.mjs` and `.cjs` extensions when compiled. However, unlike the existing `.ts` and `.tsx` extensions, expressions that start with `<` are disallowed when they would be ambiguous depending on whether they are parsed in `.ts` or `.tsx` mode. The ambiguity is caused by the overlap between the syntax for JSX elements and the old deprecated syntax for type casts.
+    The upcoming version 4.5 of TypeScript is introducing the `.mts` and `.cts` extensions that turn into the `.mjs` and `.cjs` extensions when compiled. However, unlike the existing `.ts` and `.tsx` extensions, expressions that start with `<` are disallowed when they would be ambiguous depending on whether they are parsed in `.ts` or `.tsx` mode. The ambiguity is caused by the overlap between the syntax for JSX elements and the old deprecated syntax for type casts:
 
     | Syntax                        | `.ts`                | `.tsx`           | `.mts`/`.cts`        |
     |-------------------------------|----------------------|------------------|----------------------|
@@ -29,6 +29,27 @@
 
     This release of esbuild introduces a syntax error for these ambiguous syntax constructs in `.mts` and `.cts` files to match the new behavior of the TypeScript compiler.
 
+* Do not remove empty `@keyframes` rules ([#1665](https://github.com/evanw/esbuild/issues/1665))
+
+    CSS minification in esbuild automatically removes empty CSS rules, since they have no effect. However, empty `@keyframes` rules still trigger JavaScript animation events so it's incorrect to remove them. To demonstrate that empty `@keyframes` rules still have an effect, here is a bug report for Firefox where it was incorrectly not triggering JavaScript animation events for empty `@keyframes` rules: https://bugzilla.mozilla.org/show_bug.cgi?id=1004377.
+
+    With this release, empty `@keyframes` rules are now preserved during minification:
+
+    ```css
+    /* Original CSS */
+    @keyframes foo {
+      from {}
+      to {}
+    }
+
+    /* Old output (with --minify) */
+
+    /* New output (with --minify) */
+    @keyframes foo{}
+    ```
+
+    This fix was contributed by [@eelco](https://github.com/eelco).
+
 ## 0.13.4
 
 * Fix permission issues with the install script ([#1642](https://github.com/evanw/esbuild/issues/1642))

internal/css_parser/css_parser.go

@@ -279,6 +279,11 @@ skipRule:
 		rule := rules[i]
 
 		switch r := rule.Data.(type) {
+		case *css_ast.RAtKeyframes:
+			// Do not remove empty "@keyframe foo {}" rules. Even empty rules still
+			// dispatch JavaScript animation events, so removing them changes
+			// behavior: https://bugzilla.mozilla.org/show_bug.cgi?id=1004377.
+
 		case *css_ast.RKnownAt:
 			if len(r.Rules) == 0 {
 				continue