A site defined alphabet for ad identification about web-advertising HOT 2 OPEN

If the publisher can declare a manifest w/ ad identifiers, why can't the publisher just offer a Javascript interface from "ad identifier" --> "lower entropy index" into the manifest directly in the page?

I don't see how this manifest lookup needs to be done by the browser.

from web-advertising.

The alphabet of ad identifiers would be available to external metrics servers, and unalterable by script on a page. The ad tag could be dynamically inserted into the DOM by external script loaded into the top level context, so could write arbitrary ad ids into the tag. Having an externally accessible alphabet stops that.

The Origin Policy idea is that browsers would always cache the resource making it immediately be available to script, so ordinal position lookups would be reasonably efficient.

from web-advertising.