レスポンシブウェブデザインとは何ですか？
レスポンシブウェブデザインとは、HTMLとCSSを使用してウェブサイトのサイズを自動的に変更することです。
レスポンシブウェブデザインとは、すべてのデバイス（デスクトップ、タブレット、電話）でウェブサイトの見栄えを良くすることです。
ビューポートの設定
レスポンシブWebページを作成するときは、<meta>すべてのWebページに次の要素を追加します。
例
<meta name="viewport" content="width=device-width, initial-scale=1.0">
メディアクエリ
メディアクエリは、レスポンシブWebページで重要な役割を果たします。
メディアクエリを使用すると、ブラウザのサイズごとに異なるスタイルを定義できます。
例：
次の3つの要素が大画面に水平表示と垂直方向に小さな画面であろうことを確認するために、ブラウザウィンドウのサイズを変更：
例
<style>
.left, .right {
float: left;
width: 20%; /* The width is 20%, by default */
}
.main {
float: left;
width: 60%; /* The width is 60%, by default */
}
/* Use Media Query to
add a breakpoint at 800px: */
@media screen and (max-width:800px) {
.left , .main, .right {width:100%;}
}
</style>
W3SchoolsのRWDチュートリアルでレスポンシブWebデザインの詳細をご覧ください
レスポンシブ画像
Responsive images are images that scale nicely to fit any browser size.
When the CSS width property is set to a percentage value, an image will scale up and down when resizing the browser window.
This image is responsive:
Example
<img
src="img_girl.jpg" style="width:80%;height:auto;">
If the max-width property is set to 100%, the image will scale down if it has to, but never scale up to be larger than its original size:
Example
<img src="img_girl.jpg" style="max-width:100%;height:auto;">
Image Depending on Browser Size
The HTML <picture> element allows you to define different images for
different browser window sizes.
Example
<picture>
<source srcset="img_smallflower.jpg" media="(max-width:
600px)">
<source srcset="img_flowers.jpg" media="(max-width:
1500px)">
<source srcset="flowers.jpg">
<img src="img_smallflower.jpg"
alt="Flowers">
</picture>
Responsive W3.CSS
W3.CSS is a free CSS Framework that offers Responsive Design by default.
W3.CSS makes it easy to develop sites that look nice on any device; desktop, laptop, tablet, or phone:
Example
<!DOCTYPE html>
<html>
<meta name="viewport"
content="width=device-width, initial-scale=1">
<link rel="stylesheet"
href="https://www.w3schools.com/w3css/4/w3.css">
<body>
<div class="w3-center w3-padding-64 w3-light-grey">
<h1>My W3.CSS Page</h1>
<p>Resize this page to see the responsive effect!</p>
</div>
<div
class="w3-row-padding">
<div class="w3-third">
<h2>London</h2>
<p>London is the capital city of England.</p>
<p>It is the most populous city in the United Kingdom,
with a
metropolitan area of over 13 million inhabitants.</p>
</div>
<div
class="w3-third">
<h2>Paris</h2>
<p>Paris is
the capital of France.</p>
<p>The Paris area is one of the largest
population centers in Europe,
with more than 12 million
inhabitants.</p>
</div>
<div class="w3-third">
<h2>Tokyo</h2>
<p>Tokyo is the capital of Japan.</p>
<p>It
is the center of the Greater Tokyo Area,
and the most populous
metropolitan area in the world.</p>
</div>
</div>
</body>
</html>
To learn more about W3.CSS, go to our W3.CSS Tutorial.
Bootstrap
Bootstrap is a very popular framework that uses HTML, CSS and jQuery to make responsive web pages.
Example
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap
Example</title>
<meta charset="utf-8">
<meta name="viewport"
content="width=device-width, initial-scale=1">
<link rel="stylesheet"
href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css">
<script
src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script
src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js"></script>
<script
src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js"></script>
</head>
<body>
<div class="jumbotron text-center">
<h1>My
First Bootstrap Page</h1>
<p>Resize this responsive page to see the
effect!</p>
</div>
<div class="container-fluid">
<div class="row">
<div class="col-sm-4">
<h2>London</h2>
<p>London is the capital city of England.</p>
<p>It is the most populous city in the United Kingdom,
with a
metropolitan area of over 13 million inhabitants.</p>
</div>
<div class="col-sm-4">
<h2>Paris</h2>
<p>Paris is
the capital of France.</p>
<p>The Paris area is one of the largest
population centers in Europe,
with more than 12 million
inhabitants.</p>
</div>
<div
class="col-sm-4">
<h2>Tokyo</h2>
<p>Tokyo is the capital of Japan.</p>
<p>It
is the center of the Greater Tokyo Area,
and the most populous
metropolitan area in the world.</p>
</div>
</div>
</div>
</body>
</html>
To learn more about Bootstrap, go to our Bootstrap Tutorial.