The W3 Consortium has its own online validator which you can use for free.It may be found at: https://validator.w3.org/ A CSS validator checks your Cascading Style Sheet in the same manner.Beginners and students, on the other hands, will find automated checking tools invaluable in spotting mistakes.
It is now just over a year since a court first awarded damages to a blind user against the owners of a website he found inaccessible (Maguire vs SOCOG, August 2000).
This is deliberate, and doesn't imply any kind of browser bug.
A term sometimes used for this is WYSINWOG - What You See Is Not What Others Get (unless by coincidence).
That is, it will check that it complies with the CSS standards set by the W3 Consortium.
There are a few which will also tell you which CSS features are supported by which browsers (since not all browsers are equal in their CSS implementation).