선택자에 추가하는 키워드 선택한 요소의 지정된 부분에 스타일 적용
<!DOCTYPE html>
<html lang="ko">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.required::before {
content: '*';
color: red;
}
</style>
</head>
<body>
<form action="">
<label for="id" class="required">ID</label>
<input type="text" name="id" id="id" required>
<br>
<label for="pw" class="required">PW</label>
<input type="text" name="pw" id="pw" required>
<br>
<label for="email" class="">Email</label>
<input type="text" name="email" id="email" required>
</form>
</body>
</html>

<!DOCTYPE html>
<html lang="ko">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.required::after {
content: '*';
color: red;
}
</style>
</head>
<body>
<form action="">
<label for="id" class="required">ID</label>
<input type="text" name="id" id="id" required>
<br>
<label for="pw" class="required">PW</label>
<input type="text" name="pw" id="pw" required>
<br>
<label for="email" class="">Email</label>
<input type="text" name="email" id="email" required>
</form>
</body>
</html>

<!DOCTYPE html>
<html lang="ko">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
p::first-letter {
color: red;
}
</style>
</head>
<body>
<p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Reprehenderit aliquid voluptates perspiciatis quibusdam, iure maiores eaque accusamus optio ipsa hic ducimus itaque, recusandae architecto pariatur ex blanditiis? Eveniet, tenetur iusto?</p>
</body>
</html>

## 선택자::first-letter
- 특정 요소의 첫 글자를 선택
```html
<!DOCTYPE html>
<html lang="ko">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
p::first-letter {
color: red;
}
</style>
</head>
<body>
<p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Reprehenderit aliquid voluptates perspiciatis quibusdam, iure maiores eaque accusamus optio ipsa hic ducimus itaque, recusandae architecto pariatur ex blanditiis? Eveniet, tenetur iusto?</p>
</body>
</html>
