Re: [w3c/editing] Should execCommand dispatch beforeinput or not? (#200)

> > That means that web apps may want to distinguish whether each `beforeinput` is fired by `execCommand` or not.
> 
> Web apps **do not use `execCommand`** for editing and it's not an option to use it either except for small 5-line demos.

As far as I know, some developers want to use only simple `execCommand` for making modification undo-able with browser built-in undo stack. Anyway, not point of this issue.

> > I meant that whether `beforeinput` event is dispatched or not when web apps modifies editable contents with API. I meant that if `beforeinput` event is fired only while calling some API, it's inconsistent between API.
> 
> I still don't understand what you're trying to say. It seems more inconsistent for `beforeinput` to not fire in cases where `input` event would otherwise fire.

In my understanding, `beforeinput` is suggested for that web apps can cancel or overwrite the behavior of user operation before browsers modify the DOM tree. If somebody want to "correct" characters and make it undo-able without their own transaction manager, they may want to do:

```
editor.addEventListener("beforeinput", (event) => {
  if (event.inputType === "insertText") {
    let modifiedData = modifySomething(event.data);
    if (modifiedData !== event.data) {
      document.execCommand("insertText", false, modifiedData);
      event.preventDefault();
    }
  }
});
```

> If someone is writing a library which needs to react to whenever an editing operation gets triggered in a `contenteditable` region (e.g. in order to update corresponding data model), then it needs to listen to such an event regardless of whether it happened due to direct user inputs or `execCommand`. There is no need to distinguish the two.

I don't think that `beforeinput` even is a good solution for that purpose because it cannot catch DOM tree modification by DOM API. Isn't mutation observer designed for such purpose? I think that events defined by UI Events are useful only for listening user actions.

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