Looks like painstaking work - unless you have anything in your IDE to help.

These strings can't be changed, e.g. by indenting them like

$expected1 = '
    a {
        color: green;
    }
';

... (which would be more readable) because they need to be exact.

I don't believe there is a CodeSniffer or CS-Fixer rule that would do the job (the closest I found squizlabs/PHP_CodeSniffer#1557 which would change the content of the strings).

My IDE (I think) is set to convert tabs to spaces upon save (and I'm pretty sure does it blindly), so if I edited that file I'd have broken the tests.