feat: remove @import support per new behavior in Chrome

>
> also update error messages and tests

README.md

@@ -102,3 +102,6 @@ from the `element.adoptedStyleSheets` array. The behavior here is supposed to
 emulate a `FrozenArray`, so modifying the array in question will have no effect
 until the value is changed using a setter.
 
+## A note about versioning
+
+This packages doesn't necessarily follow semantic versioning. As the spec is still under consideration and implementation by browser vendors, the features supported by this package will change (generally following Chrome's implementation).

src/ConstructStyleSheet.js

@@ -4,9 +4,7 @@ import {
   sheetMetadataRegistry,
   state,
 } from './shared';
-import {instanceOfStyleSheet} from './utils';
-
-const importPattern = /@import/;
+import {instanceOfStyleSheet, rejectImports} from './utils';
 
 const cssStyleSheetMethods = [
   'addImport',
@@ -96,34 +94,31 @@ export default class ConstructStyleSheet {
   }
 
   replace(contents) {
+    const sanitized = rejectImports(contents);
     return new Promise((resolve, reject) => {
       if (sheetMetadataRegistry.has(this)) {
         const {basicStyleElement} = sheetMetadataRegistry.get(this);
 
-        basicStyleElement.innerHTML = contents;
+        basicStyleElement.innerHTML = sanitized;
         resolve(basicStyleElement.sheet);
         updateAdopters(this);
       } else {
         reject(
           new Error(
-            "Failed to execute 'replace' on 'CSSStyleSheet': Can't call replace on non-constructed CSSStyleSheets.",
+            "Can't call replace on non-constructed CSSStyleSheets.",
           ),
         );
       }
     });
   }
 
   replaceSync(contents) {
-    if (importPattern.test(contents)) {
-      throw new Error(
-        '@import rules are not allowed when creating stylesheet synchronously',
-      );
-    }
+    const sanitized = rejectImports(contents);
 
     if (sheetMetadataRegistry.has(this)) {
       const {basicStyleElement} = sheetMetadataRegistry.get(this);
 
-      basicStyleElement.innerHTML = contents;
+      basicStyleElement.innerHTML = sanitized;
       updateAdopters(this);
 
       return basicStyleElement.sheet;

src/adopt.js

@@ -78,15 +78,20 @@ export function adoptStyleSheets(location) {
 
 export function removeExcludedStyleSheets(location, oldSheets) {
   const sheets = getAdoptedStyleSheet(location);
-
   for (let i = 0, len = oldSheets.length; i < len; i++) {
     if (sheets.indexOf(oldSheets[i]) > -1) {
       continue;
     }
 
     const {adopters} = sheetMetadataRegistry.get(oldSheets[i]);
     const observer = observerRegistry.get(location);
-    const styleElement = adopters.get(location);
+    let styleElement = adopters.get(location);
+
+    // In case the sheet was saved to document.head
+    // before the document was ready
+    if (!styleElement) {
+      styleElement = adopters.get(document.head);
+    }
 
     observer.disconnect();
     styleElement.parentNode.removeChild(styleElement);

src/utils.js

@@ -1,5 +1,7 @@
 import {adoptedSheetsRegistry, frame, OldCSSStyleSheet} from './shared';
 
+const importPattern = /@import\surl(.*?);/gi;
+
 export function instanceOfStyleSheet(instance) {
   return (
     instance instanceof OldCSSStyleSheet ||
@@ -41,3 +43,17 @@ export function getAdoptedStyleSheet(location) {
       : location,
   );
 }
+
+export function rejectImports(contents) {
+  const imports = contents.match(importPattern, '') || [];
+  let sheetContent = contents;
+  if (imports.length) {
+    console.warn(
+      '@import rules are not allowed here. See https://github.com/WICG/construct-stylesheets/issues/119#issuecomment-588352418'
+    );
+    imports.forEach(_import => {
+      sheetContent = sheetContent.replace(_import, '');
+    });
+  }
+  return sheetContent;
+}

test/polyfill.test.js

@@ -73,7 +73,7 @@ describe('Constructible Style Sheets polyfill', () => {
           .replace('.only-test { color: blue; }')
           .catch(error => {
             expect(error.message).toBe(
-              "Failed to execute 'replace' on 'CSSStyleSheet': Can't call replace on non-constructed CSSStyleSheets.",
+              "Can't call replace on non-constructed CSSStyleSheets.",
             );
           });
       });