Submenu is not aligned css html

Question

I'm having problems with my submenu. What happens is the following:

My menu has 4 links in which one of these links presents a submenu with 3 links. What is happening is that my submenu, instead of showing the links stacked one below the other, they are next to each other. I want the submenu links to be stacked.

@import url('https://fonts.googleapis.com/css2?family=Nunito:wght@200&display=swap');
            
*{
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  background-color: #FFFFFF;
}  
.header {
  background-color: #F1F1F1;
  text-align: center;
  padding: 20px;
}        
li,a,button{
  font-family: 'Nunito', sans-serif;
  font-weight: 500;
  font-size: 16px;
  color:black;
  text-decoration: none;
}        
header{
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 30px 10%;
}        
.logo{
  cursor: pointer;
}       
.nav_links{
  list-style: none;
}        
.nav_links li{
  display: inline-block;
  padding: 0px 20px;
}        
.nav_links li a{
  transition: all 0.3s ease 0s;
}

.nav_links li a:hover{
  color: #0088a9;
}        
.nav_links li ul{
  width: 200px;
  text-align: center;
  position: absolute;
  background: #373C58;
  padding: 0;
  list-style-type: none;
  box-shadow: 0px 0px 23px 0px rgba(0, 0, 0, 0.25);
  margin-top: 10px;
  border-radius: 7px;
  display: none;
}        
.nav_links li ul li{
  padding: .5rem;
}        
.nav_links li ul li a{
  width: 100%;
  height: 100%;
  display: inline-block;
  padding: 0;
  margin: 0;
}        
.nav_links li ul li:hover{
  background: #272B3F;
  border-radius: 7px;
}        
.nav_links a:focus + ul,
.nav_links a:hover + ul,
.nav_links ul:hover{
  display: block;
}        
button{
  padding: 9px 25px;
  background-color: rgba(0, 136, 169, 1.0);
  border: none;
  border-radius: 50px;
  cursor: pointer;
  transition: all 0.3s ease 0s;
}        
button:hover{
  background-color: rgba(0, 136, 169, 0.8);
}

<header>
  <img class ="logo" src="icon.png" width="100" height="50" align="left">
  <nav>
    <ul class="nav_links">
      <li><a href="#">1</a></li>
      <li><a href="#">2</a></li>
      <li>
        <a href="#">3</a>
        <ul>
          <li><a href="#">A</a></li>
          <li><a href="#">B</a></li>
          <li><a href="#">C</a></li>
        </ul>
      </li>
      <li><a href="#">4</a></li>
    </ul>
  </nav>
  <a class="login" href="#"><button>Login</button></a>
</header>

Also viewable in this fiddle

Answer 1

Since you have set .nav_links li { display: inline-block } all the li element inside .nav will inherit this attribute. In order to solve that you can do two things:

1. Instead apply the display: inline-block to all .nav_links li just add a chevron between them to indicate that you just want the display: inline-block only in the first element group. Like this: .nav_links > li { display: inline-block }.

2. As @yainspan says just include the attribute display: block in the .nav_links li ul li css selector.

Personally I do prefer the first one in order to save time and effort searching every element inside the DOM tree. But both choices are good.