Great introduction to Grid layout, but I don't know why you talked about linking css to html, because in my view, if someone doesn't know how to link css to html they don't deserve to know anything about Grid layout.
Anyway, by talking about linking css to html you make your biggest mistake which is making a mistake about the simplest things.
<h1 style color: "black"> doesn't exist anyway in html. perhaps you should try <h1 style="color:black"> that may work.
The rest of the code is quite ok, though { grid-template-columns: auto auto auto; } may create you three columns, but some thing like { grid-template-columns: 1fr, 1fr, 1fr; } or { grid-template-columns: repeat(3, 1fr); } might be easier done. I hope you can explain more about the grid attributes in your next post.