要素の横並び

displayプロパティの初期値
1. block
2. inline
3. inline-block
4. none

display: block

<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>display: block</title>
<style>
/*----------------------------------------
  reset
----------------------------------------*/
html,body {
  margin: 0;
  padding: 0;
  line-height: 1.0;
}
* {
  box-sizing: border-box;
}
a {
  text-decoration: none;
}

/*----------------------------------------
  layout
----------------------------------------*/
.container {
  max-width: 600px;
  margin: 30px auto;
}

/*----------------------------------------
  item
----------------------------------------*/
.btns {
  background-color: #e7e7e7;
  border: 1px dashed #999;
}
.btns-item {
  display: block;
  margin: 10px;
  padding: 10px;
  background-color: #fff;
  border: 1px dashed #999;
}
.btns-item a { 
  background: #67a1b8;
  color: #fff;
}
</style>
</head>
<body>
<div class="container">
  <div class="btns">
    <div class="btns-item"><a href="#">ボタン</a></div>
    <div class="btns-item"><a href="#">ボタン</a></div>
  </div>
</div><!-- /.container -->
</body>
</html>

display: inline

<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>display: inline</title>
<style>
/*----------------------------------------
  reset
----------------------------------------*/
html,body {
  margin: 0;
  padding: 0;
  line-height: 1.0;
}
* {
  box-sizing: border-box;
}
a {
  text-decoration: none;
}

/*----------------------------------------
  layout
----------------------------------------*/
.container {
  max-width: 600px;
  margin: 30px auto;
}

/*----------------------------------------
  item
----------------------------------------*/
.btns {
  background-color: #e7e7e7;
  border: 1px dashed #999;
}
.btns-item {
  display: inline;
  margin: 10px;
  padding: 10px;
  background-color: #fff;
  border: 1px dashed #999;
}
.btns-item a {
  background: #67a1b8;
  color: #fff;
}
</style>
</head>
<body>
<div class="container">
  <div class="btns">
    <div class="btns-item"><a href="#">ボタン</a></div>
    <div class="btns-item"><a href="#">ボタン</a></div>
  </div>
</div><!-- /.container -->
</body>
</html>

display: inline-block

<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>display: block</title>
<style>
/*----------------------------------------
  reset
----------------------------------------*/
html,body {
  margin: 0;
  padding: 0;
  line-height: 1.0;
}
* {
  box-sizing: border-box;
}
a {
  text-decoration: none;
}

/*----------------------------------------
  layout
----------------------------------------*/
.container {
  max-width: 600px;
  margin: 30px auto;
}

/*----------------------------------------
  item
----------------------------------------*/
.btns {
  background-color: #e7e7e7;
  border: 1px dashed #999;
}
.btns-item {
  display: inline-block;
  margin: 10px;
  padding: 10px;
  background-color: #fff;
  border: 1px dashed #999;
}
.btns-item a {
  background: #67a1b8;
  color: #fff;
}
</style>
</head>
<body>
<div class="container">
  <div class="btns">
    <div class="btns-item"><a href="#">ボタン</a></div>
    <div class="btns-item"><a href="#">ボタン</a></div>
  </div>
</div><!-- /.container -->
</body>
</html>

<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>display: block</title>
<style>
/*----------------------------------------
  reset
----------------------------------------*/
html,body {
  margin: 0;
  padding: 0;
  line-height: 1.0;
}
* {
  box-sizing: border-box;
}
a {
  text-decoration: none;
}

/*----------------------------------------
  layout
----------------------------------------*/
.container {
  max-width: 600px;
  margin: 30px auto;
}

/*----------------------------------------
  item
----------------------------------------*/
.btns {
  background-color: #e7e7e7;
  border: 1px dashed #999;
  text-align: center;
}
.btns-item {
  display: inline-block;
  /* margin: 10px; */
  background-color: #fff;
  border: 1px dashed #999;
}
.btns-item a {
  display: block;
  padding: 10px;
  background: #67a1b8;
  color: #fff;
}
</style>
</head>
<body>
<div class="container">
  <div class="btns">
    <div class="btns-item"><a href="#">ボタン</a></div>
    <div class="btns-item"><a href="#">ボタン</a></div>
  </div>
</div><!-- /.container -->
</body>
</html>