The old version (http://acid3.acidtests.org/) will reach 97/100 on modern compliant browsers. There is a newer version (https://wpt.live/acid/acid3/test.html) which incorporates the changes made to the specs in the meantime, so modern browsers will reach 100/100 on this one again. The test was carried out against this version, as you can see in the address bar in the screenshot.
Your link also only gets 99/100 on modern browsers.
Why? Well...
// test 64: more attribute tests
// attributes of the <object> element
var obj1 = document.createElement('object');
obj1.setAttribute('data', 'test.html');
// ...
assert(obj1.data.match(/^http:/), "object.data isn't absolute");
See that "http" in test 64? Turns out you'll get a score of 100/100 if you use the http version of the URL instead of HTTPS.
Interesting, by forcing http I get 100/100 with chrome on windows but the animation is not smooth (there is a pause at 69/100) and it definitely does not match the reference image (there are no colored boxes at all).
Passing the individual tests is only part of passing the complete test. Animations must be smooth and everything must be rendered pixel perfect to the sample image. All Chromium based browsers, I tried Chrome Chromium, Edge and Vivaldi appear to fail the test even if they get a 100/100 score. They all display a thin red line near the upper right of the white box. This line doesn't appear on the sample image.
