A test for the synchronicity of the "beforeunload" event was recently accepted into WPT [1]. Chrome current fails this test in violation of the specification.

Chrome Version       : 68.0.3440.75
UserAgentString: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/68.0.3440.75 Chrome/68.0.3440.75 Safari/537.36

Other browsers tested:

Chrome 68   | fail
Edge 17     | fail
Firefox 61  | pass
IE 11       | pass
Safari 11.1 | fail

[1] https://github.com/web-platform-tests/wpt/pull/12343

---

> The href attribute's setter must run these steps:
>
> 1. Parse the given value relative to the entry settings object. If that
>    failed, throw a TypeError exception.
> 2. Location-object-setter navigate to the resulting URL record. 

https://html.spec.whatwg.org/multipage/history.html#dom-location-href

> A Location object has an associated Location-object-setter navigate algorithm, which given a url, runs these steps:
>
> 1. If any of the following conditions are met, let replacement flag be unset; otherwise, let it be set:
>    - This Location object's relevant Document has completely loaded, or
>    - In the task in which the algorithm is running, an activation behavior is
>      currently being processed whose click event's isTrusted attribute is
>      true, or
>    - In the task in which the algorithm is running, the event listener for a
>      click event, whose isTrusted attribute is true, is being handled.
> 2. Location-object navigate, given url and replacement flag.

https://html.spec.whatwg.org/multipage/history.html#location-object-setter-navigate

> To Location-object navigate, given a url and replacement flag, run these steps:
>
>   1. The source browsing context is the responsible browsing context
>      specified by the incumbent settings object.
>   2. Navigate the browsing context to url, with the exceptions enabled flag
>      set. Rethrow any exceptions.
>
> If the replacement flag is set or the browsing context's session history
> contains only one Document, and that was the about:blank Document created
> when the browsing context was created, then the navigation must be done with
> replacement enabled.

https://html.spec.whatwg.org/multipage/history.html#location-object-navigate

To navigate a browsing context browsingContext to a resource resource, optionally with an exceptions enabled flag, the user agent must run these steps:

> 1. If resource is a URL, then set resource to a new request whose url is
>    resource.
> 2. If resource is a request and this is a reload-triggered navigation, then
>    set resource's reload-navigation flag.
> 3. If the source browsing context is not allowed to navigate browsingContext,
>    then:
>    [...]
> 4. If there is a preexisting attempt to navigate browsingContext, and the
>    source browsing context is the same as browsingContext, and that attempt
>    is currently running the unload a document algorithm, then return without
>    affecting the preexisting attempt to navigate browsingContext.
> 5. If the prompt to unload algorithm is being run for the active document of
>    browsingContext, then return without affecting the prompt to unload
>    algorithm.
> 6. If this is not a reload-triggered navigation, resource is a request,
>    resource's url equals browsingContext's active document's URL with exclude
>    fragments flag set, and resource's url's fragment is non-null, then
>    navigate to that fragment, with replacement enabled if this was invoked
>    with replacement enabled, and return.
> 7. Cancel any preexisting but not yet mature attempt to navigate
>    browsingContext, including canceling any instances of the fetch algorithm
>    started by those attempts. If one of those attempts has already created
>    and initialized a new Document object, abort that Document also.
>    (Navigation attempts that have matured already have session history
>    entries, and are therefore handled during the update the session history
>    with the new page algorithm, later.)
> 8. Prompt to unload the active document of browsingContext. If the user
>    refused to allow the document to be unloaded, then return.
>
>    If this instance of the navigation algorithm gets canceled while this step
>    is running, the prompt to unload algorithm must nonetheless be run to
>    completion.

https://html.spec.whatwg.org/multipage/browsing-the-web.html#navigate

> To prompt to unload, given a Document object document and optionally a
> recursiveFlag, run these steps:
>
> 1. Increase the event loop's termination nesting level by 1.
> 2. Increase the document's ignore-opens-during-unload counter by 1.
> 3. Let event be the result of creating an event using BeforeUnloadEvent.
> 4. Initialize event's type attribute to beforeunload and its cancelable
>    attribute true.
> 5. Dispatch: Dispatch event at document's Window object.

https://html.spec.whatwg.org/multipage/browsing-the-web.html#prompt-to-unload-a-document