[w3c/manifest] Update references to W3C if available (#526)

When reviewing #520, I noticed it includes a reference on line 3030 / 3148 to:
`https://dom.spec.whatwg.org/#concept-event-fire`

I suggest we update this to the equivalent in the W3C spec: 
`http://www.w3.org/TR/domcore/#firing-events`

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