DOM properties should be configurable by default

[Siegrift]

Hi,

I am working on Trusted Types (TT) polyfill which is a new API proposal for mitigating client side DOM XSS by changing the definition of dangerous DOM APIs, called "sinks".

The polyfill works by re-defining the sinks. This works for native DOM API, because properties on the DOM are configurable (see configurable section in Object.defineProperty).

I am working on an example integration of TT polyfill with domino. You can see the preliminary PR in w3c/trusted-types#299. Unfortunately, properties created by domino are non-configurable (e.g. insertAdjacentHTML) and the polyfill code breaks.

I saw a similar (closed) issue #89, for writable properties. This is a bit similar. What I propose is making the properties configurable by default. If there are no objections, I think I should have time to create a PR for this.

cc: @koto