cssに書き込んでも反応しません

前提・実現したいこと

Responsive Web Design Tutorial and ExplanationというYoutube動画のhtmlとmain.css(cssの上）とresponsive.css(cssの下）をコピペして貼り付けたのですが、自分が打ち込んだり数値を変えてみても変化がありません。どうしたら反応するでしょうか。
プログラミング初心者です。
ご回答よろしくお願いします。

該当のソースコード

html
1ソースコード<!DOCTYPE html>
2<html lang="en"><head>
3	<meta charset="utf-8">
4	<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
5	
6	<title>Introduction to Responsive Web Design</title>
7	
8	<meta name="viewport" content="width=device-width,initial-scale=1">
9	
10	<!-- css -->
11	<link rel="stylesheet" href="main.css">
12	<link rel="stylesheet" href="responsive.css" media="screen and (max-width: 900px)">
13	
14</head>
15<body>
16
17	<!-- header -->
18	<div class="header"><div class="section-inner">
19		<h1>Example Header</h1>
20	</div></div><!--/header-->
21	
22	<!-- nav -->
23	<div class="nav"><div class="section-inner">
24		<ul class="clearfix">
25			<li><a href="#">Home</a></li>
26			<li><a href="#">About</a></li>
27			<li><a href="#">Sample Link 1</a></li>
28			<li><a href="#">Sample Link 2</a></li>
29			<li><a href="#">Sample Link 3</a></li>
30			<li><a href="#">Sample Link 4</a></li>
31			<li><a href="#">Portfolio</a></li>
32			<li><a href="#">Contact</a></li>
33		</ul>
34	</div></div><!--/nav-->
35	
36	<!-- body-content -->
37	<div class="body-content"><div class="section-inner">
38		
39		<!-- thirds -->
40		<div class="thirds clearfix">
41			
42			<!-- one-third -->
43			<div class="one-third mobile-collapse">
44				<img src="images/1.jpg" alt="A bird on a fence">
45				<h2>Section One</h2>
46				<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
47			</div><!--/one-third-->
48			
49			<!-- one-third -->
50			<div class="one-third one-third-second mobile-collapse">
51				<img src="images/2.jpg" alt="A bird eating">
52				<h2>Section Two</h2>
53				<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
54			</div><!--/one-third-->
55			
56			<!-- one-third -->
57			<div class="one-third one-third-last mobile-collapse">
58				<img src="images/3.jpg" alt="A cat">
59				<h2>Section Three</h2>
60				<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
61			</div><!--/one-third-->
62			
63		</div><!--/thirds-->
64		
65		<!-- two-columns -->
66		<div class="two-columns clearfix">
67			
68			<!-- main -->
69			<div class="main mobile-collapse">
70				
71				<h2>Primary Content Section</h2>
72				
73				<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
74				
75				<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
76				
77				<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
78				
79			</div><!--/main-->
80			
81			<!-- side -->
82			<div class="side mobile-collapse">
83				
84				<!-- info-box -->
85				<div class="info-box-a">
86					<h4>Important Content</h4>
87					<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam.</p>
88				</div><!--/info-box-->
89				
90				<!-- info-box -->
91				<div class="info-box-b hide-mobile">
92					<h4>Not Vital</h4>
93					<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam.</p>
94				</div><!--/info-box-->
95				
96				<!-- info-box -->
97				<div class="info-box-b hide-mobile">
98					<h4>Not Vital</h4>
99					<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam.</p>
100				</div><!--/info-box-->
101				
102			</div><!--/side-->
103			
104		</div><!--/two-columns-->
105		
106	</div></div><!--/body-content-->
107	
108	<!-- footer -->
109	<div class="footer"><div class="section-inner">
110		
111		<p>Footer text placed here.</p>
112		
113	</div></div><!--/footer-->
114	
115
116    <div id="UMS_TOOLTIP" style="position: absolute; cursor: pointer; z-index: 2147483647; background: transparent; top: -100000px; left: -100000px;">
117	</div>
118  </body>
119  <umsdataelement id="UMSSendDataEventElement" data="https://learnwebcode.com/tutorial-files/responsive/#" docguid="null">
120  </umsdataelement>
121  <div id="TAG_ID4TOOLBAR_UMS_GUID" style="display: none;">5F26C61C-D5B8-14E2-90E0-97707012A78B</div>
122  <div id="tmtoolbar_ums_injected" style="display: none;">init_ums</div>
123</html>

css
1h1, h2, h3, h4, h5, h6, p {
2	margin: 0;
3	padding: 0;
4}
5
6p {
7	margin: 0 0 1em 0;
8	font-size: 93%;
9	line-height: 1.5em;
10}
11
12body {
13	font-family: Helvetica, Arial, sans-serif;
14	padding: 0;
15	margin: 0;
16	background-color: #3f3f3f;
17}
18
19img {
20	max-width: 100%;
21	height: auto;
22	margin: 0 0 10px 0;
23}
24
25/* Section Inner */
26div.section-inner {
27	max-width: 1100px;
28	padding: 0 25px;
29	margin: 0 auto;
30}
31
32/* Header */
33div.header {
34	background-color: #E9E9E9;
35	padding: 40px 0;
36}
37
38/* Navigation */
39div.nav {
40	background-color: #d4d4d4;
41}
42
43div.nav ul {
44	margin: 0;
45	padding: 0;
46}
47
48div.nav ul li {
49	list-style: none;
50	float: left;
51	font-size: 93%;
52}
53
54div.nav ul li a:link,
55div.nav ul li a:visited {
56	display: block;
57	padding: 10px 15px;
58	text-decoration: none;
59	color: #000;
60	border-right: 1px solid #bababa;
61}
62
63/* Body Content */
64div.body-content {
65	padding: 50px 0;
66	background-color: #FFF;
67}
68
69/* Thirds */
70div.thirds {
71	padding-bottom: 50px;
72}
73
74div.one-third {
75	width: 30%;
76	float: left;
77	margin-right: 5%;
78}
79
80div.one-third-last {
81	margin: 0;
82}
83
84/* Main Column */
85div.main {
86	width: 65%;
87	float: left;
88	margin-right: 5%;
89}
90
91/* Side Column */
92div.side {
93	width: 30%;
94	float: left;
95}
96
97/* Info Box */
98div.info-box-a {
99	background-color: #9bdaef;
100	padding: 20px;
101	font-size: 13px;
102	margin: 0 0 15px 0;
103}
104
105div.info-box-b {
106	background-color: #efdb7f;
107	padding: 20px;
108	font-size: 13px;
109	margin: 0 0 15px 0;
110}
111
112/* Footer */
113div.footer {
114	background-color: #3f3f3f;
115	color: #FFF;
116	padding: 15px 0;
117	text-align: center;
118}
119
120
121.clearfix:before, .clearfix:after { content: ""; display: table; }
122.clearfix:after { clear: both; }
123.clearfix { *zoom: 1; }