I will have a look at this. Direct link to the failing test: http://w3c-test.org/html/rendering/non-replaced-elements/flow-content-0/dialog-display.html

I believe that the test under http://w3c-test.org/html/rendering/non-replaced-elements/flow-content-0/dialog.html should be updated, as there is an assertion for display: none 

> assert_equals: display expected "none" but got "block"

Should I add the expected failures in dialog-expected.txt or file an issue at https://github.com/w3c/web-platform-tests/issues and wait until the test is updated?

That is per spec:

dialog:not([open]) { display: none; }

https://html.spec.whatwg.org/#flow-content-3

It is currently display: none in Chromium when the open attribute is absent. Changing that to display: block would be a regression.

Okay, I believe that the issue was the css-specificity. When I add display:block to dialog directly, it always overrides the dialog:not([open]) rule. It would be possible using the two rules below, but it doesn't seem particularly pretty. Just running it by you, as I want to make sure this is the way to go.

dialog:not([open]) { display: none; }
dialog:[open] { display: block; }

"dialog { display: block }" should suffice.

That results in :

external/wpt/html/rendering/non-replaced-elements/flow-content-0/dialog.html

This is a testharness.js-based test.
FAIL Closed dialog in width: 540px iframe assert_equals: display expected "none" but got "block"
PASS Open dialog in width: 540px iframe 
PASS Modal dialog in width: 540px iframe 
FAIL Closed dialog in width: 538px iframe assert_equals: display expected "none" but got "block"
PASS Open dialog in width: 538px iframe 
PASS Modal dialog in width: 538px iframe 
Harness: the test ran to completion.

The "Closed dialog" computed attribute for "display":
http://i.imgur.com/zv4HPx0.png

There's a specificity bug causing this. You can move the dialog:not([open]) rule below the dialog rule, or wait for the fix for the blocker to land.

Thanks for the fix.
https://codereview.chromium.org/2782823005/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0799d3367209d4185480b0408397eea79cd67fc8

commit 0799d3367209d4185480b0408397eea79cd67fc8
Author: martin <martin@martinrogalla.com>
Date: Thu Mar 30 04:42:34 2017

Use display:block for dialog in UA stylesheet.

This patch adds display:block for dialog to the UA stylesheet. This
modification ensures that all the tests in [3] pass.

More Information:
[1] - https://github.com/whatwg/html/pull/2458
[2] - https://github.com/w3c/web-platform-tests/pull/5211

[3] - http://w3c-test.org/html/rendering/non-replaced-elements/flow-content-0/dialog-display.html

BUG= 704848 

Review-Url: https://codereview.chromium.org/2782823005
Cr-Commit-Position: refs/heads/master@{#460665}

[delete] https://crrev.com/1582bcfa6979192f20e21d3ae4afb6951f0793c6/third_party/WebKit/LayoutTests/external/wpt/html/rendering/non-replaced-elements/flow-content-0/dialog-display-expected.txt
[modify] https://crrev.com/0799d3367209d4185480b0408397eea79cd67fc8/third_party/WebKit/LayoutTests/html/dialog/element-removed-from-top-layer-has-original-position.html
[modify] https://crrev.com/0799d3367209d4185480b0408397eea79cd67fc8/third_party/WebKit/Source/core/css/html.css

I made a chromestatus entry for this just in case.
https://www.chromestatus.com/feature/5754335753404416