Add web APIs to cli/js2/README.md (denoland#6842)

cli/js2/README.md

@@ -9,3 +9,51 @@ global scope (like in case of Deno).
 Each file is prefixed with a number, telling in which order scripts should be
 loaded into V8 isolate. This is temporary solution and we're striving not to
 require specific order (though it's not 100% obvious if that's feasible).
+
+## Deno Web APIs
+
+This directory facilities Web APIs that are available in Deno.
+
+Please note, that some implementations might not be completely aligned with
+specification.
+
+Some Web APIs are using ops under the hood, eg. `console`, `performance`.
+
+## Implemented Web APIs
+
+- [Blob](https://developer.mozilla.org/en-US/docs/Web/API/Blob): for
+ representing opaque binary data
+- [Console](https://developer.mozilla.org/en-US/docs/Web/API/Console): for
+ logging purposes
+- [CustomEvent](https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent),
+ [EventTarget](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget)
+ and
+ [EventListener](https://developer.mozilla.org/en-US/docs/Web/API/EventListener):
+ to work with DOM events
+ - **Implementation notes:** There is no DOM hierarchy in Deno, so there is no
+ tree for Events to bubble/capture through.
+- [fetch](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch),
+ [Request](https://developer.mozilla.org/en-US/docs/Web/API/Request),
+ [Response](https://developer.mozilla.org/en-US/docs/Web/API/Response),
+ [Body](https://developer.mozilla.org/en-US/docs/Web/API/Body) and
+ [Headers](https://developer.mozilla.org/en-US/docs/Web/API/Headers): modern
+ Promise-based HTTP Request API
+- [FormData](https://developer.mozilla.org/en-US/docs/Web/API/FormData): access
+ to a `multipart/form-data` serialization
+- [Performance](https://developer.mozilla.org/en-US/docs/Web/API/Performance):
+ retrieving current time with a high precision
+- [setTimeout](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/setTimeout),
+ [setInterval](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/setInterval),
+ [clearTimeout](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/clearTimeout):
+ scheduling callbacks in future and
+ [clearInterval](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/clearInterval)
+- [Stream](https://developer.mozilla.org/en-US/docs/Web/API/Streams_API) for
+ creating, composing, and consuming streams of data
+- [URL](https://developer.mozilla.org/en-US/docs/Web/API/URL) and
+ [URLSearchParams](https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams):
+ to construct and parse URLSs
+- [Worker](https://developer.mozilla.org/en-US/docs/Web/API/Worker): executing
+ additional code in a separate thread
+ - **Implementation notes:** Blob URLs are not supported, object ownership
+ cannot be transferred, posted data is serialized to JSON instead of
+ [structured cloning](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm).