Spec for @media: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/color

Confirmed with ToT build that the bug is indeed specific to OOPIFs. 

Returning 0 when MainFrame() is not a LocalFrame here:

https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/css/media_values.cc?q=CalculateColorBitsPerComponent&sq=package:chromium&dr=CSs&l=79-81

Thanks for the pointer! I took a quick look and it seems that there are two methods in this file that explicitly check for LocalFrame and return early. The rest of the methods query directly, so I tried a quick test to remove the early returns and the test page rendered fine. Here is a diff of what I tried:

diff --git a/third_party/blink/renderer/core/css/media_values.cc b/third_party/blink/renderer/core/css/media_values.cc                                                  
index c2b522a84ddc..904b91eaaa6d 100644
--- a/third_party/blink/renderer/core/css/media_values.cc
+++ b/third_party/blink/renderer/core/css/media_values.cc
@@ -76,9 +76,6 @@ int MediaValues::CalculateColorBitsPerComponent(LocalFrame* frame) {
   DCHECK(frame);
   DCHECK(frame->GetPage());
   DCHECK(frame->GetPage()->MainFrame());
-  if (!frame->GetPage()->MainFrame()->IsLocalFrame() ||
-      frame->GetPage()->GetChromeClient().GetScreenInfo().is_monochrome)
-    return 0;
   return frame->GetPage()
       ->GetChromeClient()
       .GetScreenInfo()
@@ -89,9 +86,6 @@ int MediaValues::CalculateMonochromeBitsPerComponent(LocalFrame* frame) {
   DCHECK(frame);
   DCHECK(frame->GetPage());
   DCHECK(frame->GetPage()->MainFrame());
-  if (!frame->GetPage()->MainFrame()->IsLocalFrame() ||
-      !frame->GetPage()->GetChromeClient().GetScreenInfo().is_monochrome)
-    return 0;
   return frame->GetPage()
       ->GetChromeClient()
       .GetScreenInfo()

Note that I haven't run any other tests, so it is quite possible some tests might break, though I would posit that this should be the right behavior.

Thanks for the code link.

Seems the bad behavior was introduced in https://codereview.chromium.org/336553003/. I think the monochrome checks are needed, but the mainFrame checks are the issue.

https://chromium-review.googlesource.com/c/chromium/src/+/1238542

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a3f28c81b8ceb9f8cc6edc985305c87a123007bc

commit a3f28c81b8ceb9f8cc6edc985305c87a123007bc
Author: Rune Lillesveen <futhark@chromium.org>
Date: Mon Sep 24 08:59:18 2018

Get ScreenInfo from Page's client regardless of OOPIF.

Two of the MediaValues calculations ignored ScreenInfo when called from
out-of-process iframes. There is no reason to do that. Just skip that
check.

Bug:  876760 
Change-Id: I6d5139f9bcbaaac45c7fcaa764d84b41151a93a9
Reviewed-on: https://chromium-review.googlesource.com/1238542
Reviewed-by: Anders Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593492}
[modify] https://crrev.com/a3f28c81b8ceb9f8cc6edc985305c87a123007bc/third_party/blink/renderer/core/css/media_values.cc
[modify] https://crrev.com/a3f28c81b8ceb9f8cc6edc985305c87a123007bc/third_party/blink/renderer/core/exported/web_frame_test.cc

The fix seems very simple and safe. Should we merge it to M70 to ensure it reaches users faster?