Another use case for webcompat.

Spending a lot of time to try to diagnose and understand a different behavior in between Blink/WebKit and Gecko, then to discover later that the document was in Quirks Mode.

Sending a warning in the console or showing a red flag on the wrong doctype or lack of doctype, would allow us

1. save time for diagnosis
2. encourage web developers to switch to standard mode, hence have more compatible web pages.

see for example https://github.com/webcompat/web-bugs/issues/66747

Note that Firefox already logs a console warning when <meta charset> is missing. A similar kind of warning could be logged in case of quirks mode.

Henri, would you know from where we could emit such message ? This seems like a potential place https://searchfox.org/mozilla-central/rev/3f97afc8db535f9b0232222cb48cc4cbf8334c76/parser/html/nsHtml5DocumentBuilder.cpp#91-93 but I'm not sure this handles non html5 documents?

The relevant place is in https://searchfox.org/mozilla-central/rev/3f97afc8db535f9b0232222cb48cc4cbf8334c76/parser/html/nsHtml5TreeBuilderCppSupplement.h#1385

The parser with "nsHtml5" in the name is used for all text/html, except about:blank.

This patch is reusing the errAlmostStandardsDoctype and errQuirkyDoctype strings
to display an error message in the webconsole.

A webconsole mochitest is added in the following patch of this stack.

In the previous patch of this stack, we started to emit error messages for
documents with non-standards doctype.
This patch makes it so we also add a Learn More link to the MDN page about
doctype modes, and adds a test to check that everything is displayed as
we expect.

Depends on D131889

The new warning message was messing up with some tests using doctype-less pages.

Depends on D131890

https://hg.mozilla.org/mozilla-central/rev/6e416679c215
https://hg.mozilla.org/mozilla-central/rev/c025cb7c3946
https://hg.mozilla.org/mozilla-central/rev/9ac235edd25f

Can we exclude about:blank?
Some Thunderbird tests are failing (due to the console warning) for about:blank being in quirks mode.
https://treeherder.mozilla.org/logviewer?job_id=360200830&repo=comm-central&lineNumber=5361

Check webconsole metrics, we are now loading 70 more modules, see https://treeherder.mozilla.org/perfherder/comparesubtest?framework=12&originalProject=autoland&originalSignature=3765730&newProject=autoland&newSignature=3765730&originalRevision=394b3415b476edd81fc8728abb82c25ded4a7d16&newRevision=356c521826254fbae435b212923b20801d171950&page=1

I want to run the test and check which modules are now loaded.

Here's the diff of newly loaded modules:

resource://devtools/client/shared/source-utils.js
resource://devtools/client/webconsole/components/Output/MessageContainer.js
resource://devtools/client/webconsole/components/Output/message-types/ConsoleApiCall.js
resource://devtools/client/webconsole/components/Output/GripMessageBody.js
resource://devtools/client/webconsole/utils/object-inspector.js
resource://devtools/client/shared/components/reps/index.js
resource://devtools/client/shared/components/reps/reps/constants.js
resource://devtools/client/shared/components/reps/reps/rep.js
resource://devtools/client/shared/components/reps/reps/undefined.js
resource://devtools/client/shared/components/reps/reps/rep-utils.js
resource://devtools/client/shared/components/reps/reps/null.js
resource://devtools/client/shared/components/reps/reps/string.js
resource://devtools/client/shared/components/reps/reps/number.js
resource://devtools/client/shared/components/reps/reps/array.js
resource://devtools/client/shared/components/reps/reps/object.js
resource://devtools/client/shared/components/reps/reps/prop-rep.js
resource://devtools/client/shared/components/reps/reps/symbol.js
resource://devtools/client/shared/components/reps/reps/infinity.js
resource://devtools/client/shared/components/reps/reps/nan.js
resource://devtools/client/shared/components/reps/reps/accessor.js
resource://devtools/client/shared/components/reps/reps/accessible.js
resource://devtools/client/shared/components/reps/reps/attribute.js
resource://devtools/client/shared/components/reps/reps/big-int.js
resource://devtools/client/shared/components/reps/reps/date-time.js
resource://devtools/client/shared/components/reps/reps/document.js
resource://devtools/client/shared/components/reps/reps/document-type.js
resource://devtools/client/shared/components/reps/reps/event.js
resource://devtools/client/shared/components/reps/reps/grip.js
resource://devtools/client/shared/components/reps/reps/function.js
resource://devtools/client/shared/components/reps/reps/promise.js
resource://devtools/client/shared/components/reps/reps/regexp.js
resource://devtools/client/shared/components/reps/reps/stylesheet.js
resource://devtools/client/shared/components/reps/reps/comment-node.js
resource://devtools/client/shared/components/reps/shared/dom-node-constants.js
resource://devtools/client/shared/components/reps/reps/element-node.js
resource://devtools/client/shared/components/reps/reps/text-node.js
resource://devtools/client/shared/components/reps/reps/error.js
resource://devtools/client/shared/components/reps/reps/window.js
resource://devtools/client/shared/components/reps/reps/object-with-text.js
resource://devtools/client/shared/components/reps/reps/object-with-url.js
resource://devtools/client/shared/components/reps/reps/grip-array.js
resource://devtools/client/shared/components/reps/shared/grip-length-bubble.js
resource://devtools/client/shared/components/reps/reps/grip-map.js
resource://devtools/client/shared/components/reps/reps/grip-map-entry.js
resource://devtools/client/shared/components/object-inspector/index.js
resource://devtools/client/shared/components/object-inspector/components/ObjectInspector.js
resource://devtools/client/shared/components/object-inspector/actions.js
resource://devtools/client/shared/components/object-inspector/utils/load-properties.js
resource://devtools/client/shared/components/object-inspector/utils/client.js
resource://devtools/client/shared/components/object-inspector/utils/node.js
resource://devtools/client/shared/components/object-inspector/reducer.js
resource://devtools/client/shared/components/Tree.js
resource://devtools/client/shared/components/object-inspector/components/ObjectInspectorItem.js
resource://devtools/client/shared/components/object-inspector/utils/index.js
resource://devtools/client/shared/components/object-inspector/utils/selection.js
resource://devtools/client/webconsole/components/Output/ConsoleTable.js
resource://devtools/client/webconsole/components/Output/Message.js
resource://devtools/client/webconsole/components/Output/MessageIndent.js
resource://devtools/client/webconsole/components/Output/MessageIcon.js
resource://devtools/client/shared/components/Frame.js
resource://devtools/client/webconsole/components/Output/message-types/ConsoleCommand.js
resource://devtools/client/webconsole/components/Output/message-types/CSSWarning.js
resource://devtools/client/webconsole/components/Output/message-types/DefaultRenderer.js
resource://devtools/client/webconsole/components/Output/message-types/EvaluationResult.js
resource://devtools/client/webconsole/components/Output/message-types/NetworkEventMessage.js
resource://devtools/client/netmonitor/src/utils/mdn-utils.js
resource://devtools/client/webconsole/components/Output/message-types/PageError.js
resource://devtools/client/webconsole/components/Output/message-types/SimpleTable.js
resource://devtools/client/webconsole/components/Output/message-types/WarningGroup.js

I noticed we are almost not using lazy loading in shared/components and in webconsole/components. Maybe it would be worth filing a bug to use lazy loading and fix the performance regression?

Related alert is https://treeherder.mozilla.org/perfherder/alerts?id=32609&hideDwnToInv=0

(In reply to Magnus Melin [:mkmelin] from comment #14)

Can we exclude about:blank?
Some Thunderbird tests are failing (due to the console warning) for about:blank being in quirks mode.
https://treeherder.mozilla.org/logviewer?job_id=360200830&repo=comm-central&lineNumber=5361

That would make sense. I wonder if we could exclude other pages (about:*, view-source:, …)