Website looks different in different browsers

Question

I am testing my website and I noticed that in Safari it looks different to Chrome and Mozila. Checked my html and css in validator and it all seems fine, so not sure why is this happening.

I validated html and css and found 1 mistake in HTML and 3 in CSS, although its a first attempt so its messy, i think, but what could make this happening?

Bellow is banner's css, can this linear gradient affect it?

Guys I'm total noob, its my first ever attempt, so when answer, please speak english language first of all, and than code-language :).

    .main-header {
  width: 100%;
  box-sizing: border-box;
  margin: auto;
  height: 750px;
  background:
  linear-gradient(0deg, whitesmoke, transparent 70%, rgba(200,216,239,1)),
  url('../img/MainGib.jpeg') no-repeat center;
  background-size: cover;
  text-align: center;
}

Answer 1

Not sure, but you might have to add css for -moz and -webkit:

-webkit-linear-gradient(0deg, whitesmoke, transparent 70%, rgba(200,216,239,1)),
  url('../img/MainGib.jpeg') no-repeat center;
-moz-linear-gradient(0deg, whitesmoke, transparent 70%, rgba(200,216,239,1)),
  url('../img/MainGib.jpeg') no-repeat center;