Feature detection using HTMLScriptElement.supports()

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

triggers.md

@@ -26,6 +26,7 @@ The existence of web-facing triggers doesn't necessarily preclude the user agent
     - [Extension: Handler URLs](#extension-handler-urls)
   - [Proposed Processing Model](#proposed-processing-model)
   - [Developer Tooling](#developer-tooling)
+  - [Feature detection](#feature-detection)
 - [Alternatives considered](#alternatives-considered)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
@@ -160,6 +161,30 @@ It will likely be useful to surface in developer tools what rules and URLs have
 
 This information and control is important because otherwise it may be difficult to validate correct behavior as it would otherwise depend on heuristics unknown to the author. Similarly testing tools such as [WebDriver][webdriver] should likely permit overriding the user agent's selection of which valid speculations to execute.
 
+### 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('speculationrules')` must return true.
+
+```js
+if (HTMLScriptElement.supports && HTMLScriptElement.supports('speculationrules')) {
+  console.log('Your browser supports speculationrules.');
+}
+```
+
+#### 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 "`speculationrules`", then return true.
+
+
 ## Alternatives considered
 
 The obvious alternative is to extend the `<link>` element.