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Bug in Acid3 test #72 (race condition when images decode asynchronously)

From: Daniel Holbert <dholbert@mozilla.com>
Date: Wed, 21 Dec 2016 16:13:46 -0800
To: Ian Hickson <ian@hixie.ch>, www-archive@w3.org
Message-ID: <7f1163b9-9f40-f80b-5a17-dfce7a8e5b5e@mozilla.com>
Hi Hixie,

tl;dr: here's my requested acid3 test modification:
  https://bug1325080.bmoattachments.org/attachment.cgi?id=8820958

BACKGROUND
==========
We at Mozilla got a bug report[0] for a regression in our Acid3 test
score, but in fact I think it's an incorrect/unstated dependency in the
test, and I think it should be fixed in the test itself.

This is for subtest #72 ("dynamic modification of <style> blocks' text
nodes"), and it's just an issue with one of the initial-conditions
sanity-checks in that test.

ANALYSIS/EXPLANATION:
=====================
Basically, Acid3 test #72 adds a styled <img> element to the document
(technically this happens in the "finally{}" for test #71), and then
synchronously reads img.height in JS, and asserts that it matches the
specified CSS "height: 10px": This assertion is:

  assertEquals(doc.images[0].height, 10,
    "prerequisite failed: style didn't affect image");

I claim this assertEquals() isn't entirely justified.  If the image data
isn't cached & the image-decoder hasn't kicked into action yet, then the
image will still be in its initial "not available" state, as defined in
the HTML5 spec [1]. And since the 'src' attribute is set and the 'alt'
attribute is set to the empty string, that means "the element represents
nothing" [2].  And elsewhere, the spec says an <img> element that
"represents nothing" is supposed to be rendered as an empty inline
element [3].  And the "height" CSS property has no effect on an empty
non-replaced inline element [4].  So, the assertion about "height"
applying is invalid, in this case where the image hasn't advanced past
its not-available state yet.  The assertion is only valid once the image
has become "available".

SUGGESTED FIX:
==============
Could you edit Acid3 subtest #72 to check the img's "complete" flag
(HTMLImageElement.complete), and return "retry" if that flag is false?
It looks like the harness will handle this nicely and retry the test
after a short timeout. As noted above, I've got a patch to do this here:
https://bug1325080.bmoattachments.org/attachment.cgi?id=8820958
I tested this locally in Firefox & Chrome, and it seems to work nicely.

(According to MDN, the "HTMLImageElement.complete" flag is supported in
all browsers [5].)

Thanks!
~Daniel

[0] https://bugzilla.mozilla.org/show_bug.cgi?id=1325080

[1]
https://html.spec.whatwg.org/multipage/embedded-content.html#img-available
"an image request is initially unavailable"

[2]
https://html.spec.whatwg.org/multipage/embedded-content.html#the-img-element:attr-img-src-5

[3] https://html.spec.whatwg.org/multipage/rendering.html#images-3
(Technically this chunk of spec requires that "the user agent does not
expect this to change". But the spec doesn't say *what* to do if the
user agent *does* expect this to change, or what "expects this to
change" actually means.)

[4] https://www.w3.org/TR/CSS21/visudet.html#propdef-height
"This property does not apply to non-replaced inline elements"

[5]
https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement#Browser_compatibility
Received on Thursday, 22 December 2016 00:14:16 UTC

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