Proposal

partial interface document {
  readonly attribute FontList fonts;
}

interface FontList : EventTarget {
  /* whatever idl magic you need to make this an array-like filled with Font objects */
  Future ready()
  boolean checkFont(DOMString font, optional DOMString text = " ");
  Future loadFont(DOMString font, optional DOMString text = " ");
  attribute EventHandler onloading;
  attribute EventHandler onloadingdone;
}

interface Font {
  Future ready()
  /* readonly versions of all the CSSFontFaceRule attributes, minus ones that point into the stylesheet */
}

First of all, I'm replacing document.fontLoader with document.fonts, an array-like of Font objects. Each Font object is a readonly version of a CSSFontFaceRule object, without any links into the CSSOM. This seems like a useful convenience API all by itself, but it's also something that can be safely exposed to Workers, as there's no DOM/CSSOM link to make it unsafe. The FontList contains Font objects for all the @font-face rules in all the stylesheets for the document, in document order.

The FontList.ready() function returns a Future. This is initially unresolved, and resolves when the browser has loaded all stylesheets and has finished loading all the fonts it chooses to initially load. It must return the same future across multiple calls. If, after resolving the future, the browser beings loading more fonts, it must create a new future to return from .ready(), and again resolve it when the browser stops loading fonts. This addresses use-case #1, replacing notifyWhenFontsReady().

The loadFont() function has its argument list reverted to be identical to checkFont, and it just returns a future which is resolved as soon as all the necessary requested fonts have loaded (which may be immediately, if all necessary fonts are already loaded). This addresses use-case #2.

checkFont() is unchanged.

To address use-case #3, the API has two pieces. If you care about the loading status of individual fonts, look for the relevant Font object and call its ready() function to obtain a Future which is resolved when the font is done loading, or cancelled when the font load has an error. This replaces the loadstart/load/error events. The loading/loadingDone events are unchanged - they're useful for providing UI indicating whether fonts are loading or not, rather than a one-off “tell me when fonts are ready”, which is what the .ready() future is for.