fix: css sourcemap generation with unicode filenames (#9120)

* fix: css sourcemap generation with unicode filenames

* format

.changeset/eighty-poems-deliver.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: css sourcemap generation with unicode filenames

packages/svelte/src/compiler/utils/mapped_code.js

@@ -292,7 +292,18 @@ export function apply_preprocessor_sourcemap(filename, svelte_map, preprocessor_
 		toUrl: {
 			enumerable: false,
 			value: function toUrl() {
-				return 'data:application/json;charset=utf-8;base64,' + btoa(this.toString());
+				let b64 = '';
+				if (typeof window !== 'undefined' && window.btoa) {
+					// btoa doesn't support multi-byte characters
+					b64 = window.btoa(unescape(encodeURIComponent(this.toString())));
+				} else if (typeof Buffer !== 'undefined') {
+					b64 = Buffer.from(this.toString(), 'utf8').toString('base64');
+				} else {
+					throw new Error(
+						'Unsupported environment: `window.btoa` or `Buffer` should be present to use toUrl.'
+					);
+				}
+				return 'data:application/json;charset=utf-8;base64,' + b64;
 			}
 		}
 	});