I am practicing responsive web design, and using flex.
The problem is, I want to make the width of the .menu li 100% when the screen is smaller than 768px, so I tried to give .menu ul and .menu li width: 100%. But it doesn't work. I also tried flex: 0 0 100% but nothing happened.
How do I make the .menu li 100%?
* {
  box-sizing: border-box;
}
nav ul,
.outline ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
}
/*common*/
/*layout*/
.container {
  max-width: 900px;
  margin: 0;
}
.outline {
  flex: 2;
}
section {
  flex: 7;
}
/*header*/
header {
  display: flex;
}
.login ul,
.menu ul {
  display: flex;
}
.login ul {
  justify-content: flex-end;
}
.toparea {
  margin-left: auto;
}
/*section*/
.contents {
  display: flex;
}
/*media query*/
@media only screen and (max-width: 768px) {
  body {
    background-color: #EAC0F0;
  }
  header {
    flex-direction: column;
  }
  .logo {
    justify-content: center;
  }
  .login {
    display: none;
  }
  .menu ul {
    flex-direction: column;
  }
  .contents {
    flex-direction: column;
  }
  .outline ul {
    display: flex;
  }
  .outline li {
    flex: 1;
  }
}
/*footer*/
/*temp*/
img,
.login ul,
.menu ul,
.outline li {
  border: 1px solid black;
}<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>RWD</title>
  <link rel="stylesheet" href="./rwd.css">
</head>
<body>
  <div class="container">
    <header>
      <img src="" alt="logo">
      <div class="toparea">
        <nav class="login">
          <ul>
            <li>Login</li>
            <li>Help</li>
          </ul>
        </nav>
        <nav class="menu">
          <ul>
            <li>Menu1</li>
            <li>Menu2</li>
            <li>Menu3</li>
            <li>Menu4</li>
          </ul>
        </nav>
      </div>
    </header>
    <div class="contents">
      <div class="outline">
        <ul>
          <li>Outline1-1</li>
          <li>Outline1-2</li>
        </ul>
      </div>
      <section>
        <h1>heading</h1>
        <p>paragraph</p>
      </section>
    </div>
    <footer>
      <address>Contact : [email protected]</address>
    </footer>
  </div>
</body>
</html>The main issue here is the margin-left: auto in the toparea rule
If you want to keep the items stacked vertically, you update your media query like this, and reset the margin margin-left: 0
@media only screen and (max-width: 768px ) { 
    body { background-color: #EAC0F0; }
    header { flex-direction: column; }
    .logo { justify-content: center; }
    .login { display: none; }
    .menu ul { flex-direction: column; }
    .contents { flex-direction: column; }
    .outline ul { display: flex; }
    .outline li {  flex: 1; }
    .toparea { margin-left: 0; }                    /*  added  */
}
Stack snippet
* { box-sizing: border-box; }
nav ul, .outline ul { list-style-type: none; margin: 0; padding: 0; }
/*common*/
/*layout*/
.container { max-width: 900px; margin: 0; }
.outline { flex: 2; }
section { flex: 7; }
/*header*/ 
header { display: flex; }
.login ul, .menu ul { display: flex; }
.login ul { justify-content: flex-end; }
.toparea { margin-left: auto; }
/*section*/
.contents { display: flex; }
/*media query*/
@media only screen and (max-width: 768px ) { 
    body { background-color: #EAC0F0; }
    header { flex-direction: column; }
    .logo { justify-content: center; }
    .login { display: none; }
    .menu ul { flex-direction: column; }
    .contents { flex-direction: column; }
    .outline ul { display: flex; }
    .outline li {  flex: 1; }
    .toparea { margin-left: 0; }               /*  added  */
}
/*footer*/
/*temp*/
img, .login ul, .menu ul, .outline li { border: 1px solid black; }<header>
  <img src="" alt="logo">
  <div class="toparea">
    <nav class="login">
      <ul>
        <li>Login</li>
        <li>Help</li>
      </ul>
    </nav>
    <nav class="menu">
      <ul>
        <li>Menu1</li>
        <li>Menu2</li>
        <li>Menu3</li>
        <li>Menu4</li>
      </ul>
    </nav>
  </div>
</header>
<div class="contents">
  <div class="outline">
    <ul>
      <li>Outline1-1</li>
      <li>Outline1-2</li>
    </ul>
  </div>
  <section>
    <h1>heading</h1>
    <p>paragraph</p>
  </section>
</div>
<footer>
  <address>Contact : [email protected]</address>
</footer>
</div>If you want them to stack horizontally, you update your media query like this, where  you let the li grow
@media only screen and (max-width: 768px ) { 
    body { background-color: #EAC0F0; }
    header { flex-direction: column; }
    .logo { justify-content: center; }
    .login { display: none; }
    .menu li { flex-grow: 1; }                    /*  changed  */
    .contents { flex-direction: column; }
    .outline ul { display: flex; }
    .outline li {  flex: 1; }
    .toparea { margin-left: 0; }                  /*  added  */
}
Stack snippet
* { box-sizing: border-box; }
nav ul, .outline ul { list-style-type: none; margin: 0; padding: 0; }
/*common*/
/*layout*/
.container { max-width: 900px; margin: 0; }
.outline { flex: 2; }
section { flex: 7; }
/*header*/ 
header { display: flex; }
.login ul, .menu ul { display: flex; }
.login ul { justify-content: flex-end; }
.toparea { margin-left: auto; }
/*section*/
.contents { display: flex; }
/*media query*/
@media only screen and (max-width: 768px ) { 
    body { background-color: #EAC0F0; }
    header { flex-direction: column; }
    .logo { justify-content: center; }
    .login { display: none; }
    .menu li { flex-grow: 1; }
    .contents { flex-direction: column; }
    .outline ul { display: flex; }
    .outline li {  flex: 1; }
    .toparea { margin-left: 0; }
}
/*footer*/
/*temp*/
img, .login ul, .menu ul, .outline li { border: 1px solid black; }<header>
  <img src="" alt="logo">
  <div class="toparea">
    <nav class="login">
      <ul>
        <li>Login</li>
        <li>Help</li>
      </ul>
    </nav>
    <nav class="menu">
      <ul>
        <li>Menu1</li>
        <li>Menu2</li>
        <li>Menu3</li>
        <li>Menu4</li>
      </ul>
    </nav>
  </div>
</header>
<div class="contents">
  <div class="outline">
    <ul>
      <li>Outline1-1</li>
      <li>Outline1-2</li>
    </ul>
  </div>
  <section>
    <h1>heading</h1>
    <p>paragraph</p>
  </section>
</div>
<footer>
  <address>Contact : [email protected]</address>
</footer>
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