"Acid3, in particular, contains some controversial tests and no longer reflects the consensus of the Web standards it purports to test, especially when it comes to issues affecting mobile browsers. The tests remain available for historical purposes and for use by browser vendors. It would be inappropriate, however, to use them as part of a certification process, especially for mobile browsers."

Wikipedia gives this as an example of a standard that changed: https://github.com/whatwg/dom/issues/319

Edit: there's an updated version at http://wpt.live/acid/acid3/test.html

And the updated one is in https://github.com/web-platform-tests/wpt/tree/master/acid for those wanting to look at the history (though it only includes the ppst-fork history, not the earlier updates).

To expand a bit more:

> Wikipedia gives this as an example of a standard that changed: https://github.com/whatwg/dom/issues/319

That's the reason why tests 23 and 25 fail (both because of the "wrong" exception being thrown); https://github.com/web-platform-tests/wpt/commit/3cbdbaa7ca9... is the Acid3 change for that.

The other test that fails, test 35, is down to https://www.w3.org/Style/CSS/Tracker/issues/317; https://github.com/web-platform-tests/wpt/commit/e6f63a6e69e... is the Acid3 change for that.

(The reason they're simply commented out is it follows the precedent set by Hixie when he was still maintaining Acid3, c.f., https://github.com/web-platform-tests/wpt/blob/master/acid/a..., plus there are other feature-specific tests for those features in WPT.)

> A quick note about the score: Acid3 dates back to 2008 and the web platform specs have changed over time so a perfect score, using the version hosted on acidtests.org, is now only 97/100. The version hosted on the web platform tests site has been updated to match the specs and so, using that, browsers should score 100/100. Similarly, on all browsers, Acid2 on a retina screen renders slightly differently, so the image above was taken on a non-retina display.

From the original blog post about Flow.

The web evolves, and some features are deliberately changed in backwards incompatible ways - eg. third party cookies, cross site resources, no audio before user interaction, removal of cross-thread arraybuffers, etc.

Browser makers seem to have this idea that if a feature is rarely used, it's fine to redefine or break it in the name of simplicity...

Browsers are user agents; the changes you mention all seem to the benefit of user experience and security. Standards are codification of things browsers have already done, or have attempted to do.