http://acid3.acidtests.org/support-a.png is a resource that page attempts to load that 404's now. I'm guessing that's part of the reason it doesn't score 100%. Not the browser's fault.
--2019-01-18 13:46:40-- http://acid3.acidtests.org/support-a.png
Resolving acid3.acidtests.org (acid3.acidtests.org)... 75.119.197.251
Connecting to acid3.acidtests.org (acid3.acidtests.org)|75.119.197.251|:80... connected.
HTTP request sent, awaiting response... 404 Not Found
2019-01-18 13:46:40 ERROR 404: Not Found.
It's actually sending a PNG file, but with a 404 response.