They're better than strict standard test suites, because they exercize real world useful stuff AND give a fun visual representation of the maturity. Wish we had such "acid test" for later CSS features like Flexbox too...
I'm talking about easy stuff, too, like normal flow, margin, border, padding box alignment.
The existing official test suites are entirely manual.
Edit: What you're describing requires you to make the entire world first, then finally test something.
Here's an example: I want to test the white-space property. How do I test that? Oh also, font rasterizers are all different and standards don't dictate what happens after box layout. It's acceptable that glyphs render with different dimensions.
How do I test that? A casual HN reader isn't going to care. Someone actually writing this stuff, will.
Rather, those acid tests were tests that test the rendering engine (in multiple aspects at the same time), and are meant for the end users: to give them a nice visual representation of the browser's progress, or lack thereof, and to push browser developers to fix issues that cause visual glitches (with features drawing the test image selected to be nice for layout writers to have) and have them race each other to pass them...
What about W3C's web-platform-tests suite [1]?
[1]: https://github.com/web-platform-tests/wpt/tree/master/css
Do it again with different CSS settings.