Feature detection using HTMLScriptElement.supports()

The HTMLScriptElement.supports() method was introduced by
whatwg/html#7008.
HTMLScriptElement.supports('importmap') must return true when supported.

Fixes WICG#171

README.md

@@ -16,6 +16,7 @@ _Or, how to control the behavior of JavaScript imports_
     - [General URL-like specifier remapping](#general-url-like-specifier-remapping)
     - [Mapping away hashes in script filenames](#mapping-away-hashes-in-script-filenames)
     - [Remapping doesn't work for `<script>`](#remapping-doesnt-work-for-script)
+    - [Feature detection](#feature-detection)
   - [Scoping examples](#scoping-examples)
     - [Multiple versions of the same module](#multiple-versions-of-the-same-module)
     - [Scope inheritance](#scope-inheritance)
@@ -282,6 +283,29 @@ would not: in all classes of browsers, it would attempt to fetch `app.mjs` direc
 <script type="module">import "./app.mjs";</script>
 ```
 
+### Feature detection
+
+If the browser supports [HTMLScriptElement](https://html.spec.whatwg.org/multipage/scripting.html#htmlscriptelement)'s
+[supports(type)](https://html.spec.whatwg.org/multipage/scripting.html#dom-script-supports) method,
+`HTMLScriptElement.supports('importmap')` must return true.
+
+```js
+if (HTMLScriptElement.supports && HTMLScriptElement.supports('importmap')) {
+  console.log('Your browser supports import maps.');
+}
+```
+
+#### Monkeypatch HTMLScriptElement.supports() method in HTML spec
+
+In the HTML spec's [HTMLScriptElement](https://html.spec.whatwg.org/multipage/scripting.html#htmlscriptelement)'s
+[supports(type)](https://html.spec.whatwg.org/multipage/scripting.html#dom-script-supports) method, before 
+
+> 3. Return false.
+
+add the following step:
+
+3. If type is "`importmap`", then return true.
+
 ### Scoping examples
 
 #### Multiple versions of the same module