"Printing steps" should not run in parallel

[domenic]

https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#printing

When the print() method is invoked, if the Document is ready for post-load tasks, then the user agent must run the printing steps in parallel. Otherwise, the user agent must only set the print when loaded flag on the Document.

As pointed out by @TimothyGu in #4682, it seems like there are several steps inside the printing steps that should be run from a the main thread, not from in-parallel. E.g. steps 3 and 4 fire an event; and step 4 may pause (and pause seems to explicitly pause a task).

One thing to test here is whether beforeprint fires synchronously when calling print(). If that is true, then the solution is:

• Remove the "in parallel" from the above text.
• In https://html.spec.whatwg.org/multipage/parsing.html#the-end:print-when-loaded, queue a task to run the printing steps, instead of running them directly.

If that is not true, then the solution is a bit more complicated.

[TimothyGu]

Seems like we are in the lucky case. On Firefox and Chrome (not sure about Safari), everything seems to be synchronous. In particular, the following

onbeforeprint = () => { console.log('beforeprint'); };
onafterprint  = () => { console.log('afterprint');  };
console.log('before calling print()');
window.print();
console.log('after print() returns');

prints

before calling print()
beforeprint
afterprint
after print() returns

in both Chrome and Firefox. I'll go ahead and create a PR.

[zcorpan]

WebKit seems to not support the events. https://bugs.webkit.org/show_bug.cgi?id=19937

cc @cdumez