Someone found the related issue already on SO: Should I use single or double colon notation for pseudo-element css
Basically, if any selector in your comma separated list is not recognized by your browser, it skips the whole rule and goes on to the next.
// Chrome
::-moz-placeholder // <-- I don't know this one, skipping!
// Firefox
::-moz-placeholder, ::webkit-input-placeholder // <-- I don't know this one, skipping!
But if you seperate them in single statements, it won't skip the one it knows.
So, as a rule of thumb: if you have vendor-prefixes, always split them up into seperate rules.