error on empty css declarations - fixes #3801

src/compiler/parse/read/style.ts

@@ -47,6 +47,13 @@ export default function read_style(parser: Parser, start: number, attributes: No
 				}
 			}
 
+			if (node.type === 'Declaration' && node.value.type === 'Value' && node.value.children.length === 0) {
+				parser.error({
+					code: `invalid-declaration`,
+					message: `Declaration cannot be empty`
+				}, node.start);
+			}
+
 			if (node.loc) {
 				node.start = node.loc.start.offset;
 				node.end = node.loc.end.offset;

test/validator/samples/invalid-empty-css-declaration/errors.json

@@ -0,0 +1,12 @@
+[{
+	"code": "invalid-declaration",
+	"message": "Declaration cannot be empty",
+	"start": {
+		"line": 11, "column": 0, "character": 93
+	},
+  	"end": {
+		"line": 11, "column": 0, "character": 93
+	},
+	"pos": 93,
+	"frame": " 9:     color: blue;\n10:   }\n11: </style>\n    ^"
+}]

test/validator/samples/invalid-empty-css-declaration/input.svelte

@@ -0,0 +1,11 @@
+<div class='foo bar'></div>
+
+<style>
+	.foo {
+		color:;
+	}
+	.bar {
+		font:;
+		color: blue;
+	}
+</style>