I was also seeing only 97% in Firefox.

Disabling the LastPass add-on brought it to 100%. I'm guessing you also use LastPass?

That's a very fingerprintable observation.

Most rendering oddities are not directly detectable by JS on the page, with the notable exception of the <canvas> element.

Though I suppose rendering differences that affect z-order or visibility could be detected indirectly, by listening for pointer events on elements that are "supposed to be" visible/hidden.

Isn't one of the main uses of JS to get the contents of elements on the page? That "97%" and "100%" would be clearly evident.

You are of course correct. I should not try to think about software at 4am! :)

You're right, disable LastPass and I get a 100%... Strange.

I get 100% using Firefox 104.0.2 (64-bit) on Windows 10. Maybe it's related to the platform or addons? Could it be uMatrix or something blocking some kind of behavior that would be questionable on a normal webpage?

I get 100% also, using Librewolf fork of FF (104.0.2). RFP is on and uBlock Origin enabled.

edit: MacOS and Windows.

Also 100% on Firefox Mobile 104.2.0

100% on FF 104.0 for Ubuntu

Try disabling addons. 10ten reader, for example, is known to alter pages' style and to break Acid3. There may be other addons doing the same.

100/100 on my Firefox, and it's fairly customized with a bunch of extensions and non-default settings to the extent some sites refuse to work normally and I have to either ignore them or open them in an "alternative" "clean" browser.

Huh. I get 100% with 104.0.2 on Ubuntu.

100% - Firefox 103.0.2 on Mac OS

99% :-/

Try “https”?

Strangely I get 99% with https and 100% with http

On FF for Android it hits 100% but the image is incorrect (there is red text in the top-left).

> Strangely I get 99% with https and 100% with http

Same (FF 104.0.2 on Mac, with uBO).

The failing test is:

    Test 64 failed: object.data isn't absolute

That test is actually assuming the url is http, and doesn't work on https.