Imagine that anybody posted a web code in which css is in standard syntax..... Does that mean some users can't run this code.

Question

Some browsers support different CSS syntax.....

Answer 1

Use https://autoprefixer.github.io/

Answer 2

css is standarized, and most of browsers are following the current standard when upgrading theirs.
so as long as we follow the standard there wont be any problem.
its different case if we use certain browser only feature. like this one
https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Scrollbars#Browser_compatibility

or the browser isnt up to date