in each browser there is something called Browser Engine, this is the heart of any browser. it take the CSS, HTML, Javascript ...etc and drowning into browser screen. it is like a compiler on programming language.
So we have one CSS code and different Browser Engine, some Engine drowning the CSS code with different way than the other engine do, and some Engine support some CSS features and the other not. here is the issue and the big headache for every Front-end developer.