how to write css and html in same page code example

Example 1: how to link css to html

<link rel="stylesheet" href="styles.css">

Example 2: how to add css to html

<link rel="stylesheet" href="hi.css">

Example 3: how to use css

<link rel="stylesheet" href="stylesheet.css">
<div class="mydiv"></div>
<style>
.mydiv {
background-color: /*Color*/;
color: /*Another color*/;
}
</style>