I am working on this isue.

document.all is a DOM API, but the implementation-wise it's area of
Blink's bindings generator?

https://chromium-review.googlesource.com/c/823513/
It looks like bhagirathi cannot be an owner for this, tentatively
setting yukishiino@ (already reviewing his patch) as a tentative owner.

In Firefox 55.0.3 -
document.all = {} // Does not work -
document.all // HTMLAllCollection
Object.defineProperty(document, 'all', {}) // Works -
document.all // undefined

Just make sure everyone is aligned on how it should work.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/74c78412e6d3839adb217ce05556f4f3c681dbf3

commit 74c78412e6d3839adb217ce05556f4f3c681dbf3
Author: Bhagirathi Satpathy <bhagirathi.s@samsung.com>
Date: Tue Dec 19 16:54:07 2017

document.all must not be replaced as per specification

document.all must not be overwritten with something else.
E.g., below javascript should not set 'document.all' to '1' instead
it should be always [object HTMLAllCollection].
<script>
document.all = 1;
</script>

Specification:
https://html.spec.whatwg.org/multipage/obsolete.html#other-elements,-attributes-and-apis
https://html.spec.whatwg.org/multipage/obsolete.html#dom-document-all

Bug:  794433 
Change-Id: I40122d7107206fe2fb680fa221cf0ae003379ea9
Reviewed-on: https://chromium-review.googlesource.com/823513
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Bhagirathi Satpathy <bhagirathi.s@samsung.com>
Cr-Commit-Position: refs/heads/master@{#525058}
[modify] https://crrev.com/74c78412e6d3839adb217ce05556f4f3c681dbf3/extensions/renderer/resources/platform_app.js
[modify] https://crrev.com/74c78412e6d3839adb217ce05556f4f3c681dbf3/third_party/WebKit/LayoutTests/external/wpt/html/dom/interfaces-expected.txt
[modify] https://crrev.com/74c78412e6d3839adb217ce05556f4f3c681dbf3/third_party/WebKit/LayoutTests/fast/dom/undetectable-document-all-expected.txt
[modify] https://crrev.com/74c78412e6d3839adb217ce05556f4f3c681dbf3/third_party/WebKit/LayoutTests/fast/dom/undetectable-document-all.html
[modify] https://crrev.com/74c78412e6d3839adb217ce05556f4f3c681dbf3/third_party/WebKit/LayoutTests/virtual/stable/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/74c78412e6d3839adb217ce05556f4f3c681dbf3/third_party/WebKit/LayoutTests/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/74c78412e6d3839adb217ce05556f4f3c681dbf3/third_party/WebKit/Source/core/dom/Document.idl

AFAICS, Gecko still declares all as

    readonly attribute HTMLAllCollection all;

while WebKit has somewhat-recently made it a readonly attribute (but a [Replaceable] one) in https://bugs.webkit.org/show_bug.cgi?id=173444. I also checked Edge 16, but 'document.all' always returns undefined in my tests, even though "'all' in document" returns true.

It'd be good to try to reproduce Phistuck's tests on Blink, I guess the different effects are due to WebIDL's rules that I keep forgetting.

In any case, it's probably a good idea to add a chromestatus.com entry about this change so that people aren't caught off-guard.

Hi Raphael, with this patch now |document.all| behavior is same as Gecko to match the specification. I do not have permission to add an entry in chromestatus.com about this change. I had requested "edit" access in the past but still not approved yet. Can anyone please add an entry in chromestatus.com for this change.
Thanks!

Filed an entry at:
https://www.chromestatus.com/feature/5072231356956672