Link to all HTML-related tests from html/README.md

Part of whatwg/html#3014.

html/README.md

@@ -4,4 +4,18 @@ This directory contains tests for [HTML](https://html.spec.whatwg.org/multipage/
 
 Sub-directory names should be based on the URL of the corresponding part of the
 multipage-version specification. For example, The URL of
-"8.3 Base64 utility methods" is [https://html.spec.whatwg.org/multipage/webappapis.html#atob](https://html.spec.whatwg.org/multipage/webappapis.html#atob). So the directory in WPT is [webappsapis/atob/](webappsapis/atob).
+"8.3 Base64 utility methods" is [https://html.spec.whatwg.org/multipage/webappapis.html#atob](https://html.spec.whatwg.org/multipage/webappapis.html#atob). So the directory in WPT is [webappapis/atob/](/html/webappapis/atob).
+
+For historical reasons, parts of HTML have their own directories:
+
+* [/2dcontext](/2dcontext)
+* [/custom-elements](/custom-elements)
+* [/eventsource](/eventsource)
+* [/imagebitmap-renderingcontext](/imagebitmap-renderingcontext)
+* [/innerText](/innerText)
+* [/offscreen-canvas](/offscreen-canvas)
+* [/shadow-dom](/shadow-dom)
+* [/webmessaging](/webmessaging)
+* [/websockets](/websockets)
+* [/webstorage](/webstorage)
+* [/workers](/workers)