It's hard to see what you're doing because you didn't properly mark your HTML and CSS context as pre-formatted text, so it's largely missing in the post above.
When posting things like this you should either select all of the (e.g. HTML or CSS) text and press the
</> button to tell the forum that it's preformatted text, or wrap the text in "code fences" to tell the forum that it should treat the text as special. As an example, code fences would look like this as you enter the text:
That said, if you had a file named
style.css which contains the text:
And an HTML file named
test.html which contains the HTML:
<link href="style.css" rel="stylesheet" type="text/css">
As long as both files are present in the same local folder, using
File > Open in your browser (e.g. Chrome or Safari) you should see an empty window with a yellow background, which proves that the HTML page is being rendered using the linked CSS.