The CSS won't connect with the HTML

I have a HTML file and a CSS file, I used the code <link type="text/css" rel="stylesheet" href="stylesheet.css"/> but it won’t work. Here is the HTML part.

<!DOCTYPE html>
<link type="text/css" rel="stylesheet" href="stylesheet.css"/>

<img src="CASA.png">
<dt><a href="aboutus.html">About Us</a></dt>
<dd>Know what is our goals!</dd>
<dt><a href="newsupdates.html">News Updates</a></dt>
<dd>Find out about the new news and the contreversies!</dd>
<dt><a href="calendarofactivities.html">Calendar of Activities</a></dt>
<dd>Want to know what projects and events that are happening soon?</dd>
<dt><a href="alumniinformation.html">Alumni Information</a></dt>
<dd>Know the experience and fun in the School from the Alumni!</dd>
<dt><a href="homeworkhelp.html">Homework Help</a></dt>
<dd>Having Trouble with the homework? Come to the Homework help to get GOOD GRADES!</dd>
<dt><a href="contactus.html">Contact Us</a></dt>
<dd>Contact us here for inquiries!</dd>
<dt><a href="departmentpages.html">Department Pages</a></dt>
<dd>About the Department.... :)</dd>

and here is the CSS,

body {
background-size:100%; }

You’ve written the background-image property incorrectly, it should be written as:
background-image: url("background.jpg");

You are not meant to write any HTML outside the <html> tags. It will not be rendered, nor will HTML be visually added to the page if not embedded in the <body> tags.