fix: add mutation observer only to head not body (#452)

README.md

@@ -297,13 +297,13 @@ In polyfill mode, multiple import maps are supported.
 Support for dynamically injecting import maps with JavaScript via e.g.:
 
 ```js
-document.body.appendChild(Object.assign(document.createElement('script'), {
+document.head.appendChild(Object.assign(document.createElement('script'), {
   type: 'importmap',
   innerHTML: JSON.stringify({ imports: { x: './y.js' } }),
 }));
 ```
 
-is also provided using mutation observers.
+is also provided using mutation observers. Note, only `document.head` appending of scripts, importmaps and preloads is supported - we do not observe body mutations to `document.body` for performance reasons.
 
 The caveat for multiple import map support polyfill support in browsers that only support a single import map is per the usual "polyfill rule" for es-module-shims - only those top-level graphs with static import feailures can be polyfilled.
 

src/es-module-shims.js

@@ -250,8 +250,8 @@ function attachMutationObserver() {
       }
     }
   });
+  observer.observe(document, { childList: true });
   observer.observe(document.head, { childList: true });
-  observer.observe(document.body, { childList: true });
   processScriptsAndPreloads();
 }
 

test/shim.js

@@ -425,8 +425,8 @@ suite('Errors', function () {
       type: 'importmap-shim',
       innerHTML: JSON.stringify(importMap),
     });
-    document.body.appendChild(script);
-    return () => document.body.removeChild(script);
+    document.head.appendChild(script);
+    return () => document.head.removeChild(script);
   }
 });