Q:

Space between the items in   in responsive design

I have made a simple menu but the list items cannot be seen properly in portrait view. They overlap each other.
I tried to add font-size: 0; to the <ul> but that doesn't change anything.
How can I create spaces between the list items in mobile view when they are vertical? 

ul#minimenu {
  padding: 0;
  margin-left: 8%;
}
ul#minimenu li {
  display: inline;
}
ul#minimenu li a {
  background-color: green;
  color: white;
  padding: 10px 10px;
  text-decoration: none;
  border-radius: 4px 4px 0 0;
}
ul#minimenu li a:hover {
  background-color: orange;
}
/*-------------------mobile-------------------*/

@media screen and (max-width: 900px) {
  ul#minimenu li {
    display: block;
  }
}
<br>
<DIV style="text-align : center;">

  <ul id="minimenu">
    <li>
      <B>levels :</B>
    </li>
    <li><a href="level1.html">Level 1</a>
    </li>
    <li><a href="level2.html">Level 2</a>
    </li>
    <li><a href="level3.html" style="background-color: #c0c0c0; color:      black;padding: 10px 10px;text-decoration: none;border-radius: 4px 4px 0 0;cursor:default;">Level 3</a>
    </li>

  </ul>

A:

how are you?
I have made some changes in the html structure / semantics because some tags are deprecated and inline css is not recommended.
It is possible to be done in several ways, I made two of them so that you can choose the best one for you.
V2 - (Recommended)
http://codepen.io/CesarCEARA/pen/OWggJX http://codepen.io/CesarCEARA/pen/OWggJX

ul#minimenu {
  display: inline-block;
  list-style: none;
  padding: 0;
}
ul#minimenu li {
  float: left;
  margin-right: 5px;
}
ul#minimenu li a {
  background-color: green;
  border-radius: 4px 4px 0 0;
  color: #fff;
  padding: 10px;
  text-decoration: none;  
}
ul#minimenu li a:hover {
  background-color: orange;
}
ul#minimenu li a.disabled {
  background-color: #c0c0c0;
  color: #000;
  cursor: default;
}
/*-------------------mobile-------------------*/
@media screen and (max-width: 900px) {
  ul#minimenu li {
    clear: both;
    display: flex;
    margin-right: 0;
    margin-bottom: 5px;
  }
  ul#minimenu li:last-child {
    margin-bottom: 0;
  }
}
<div style="text-align: center;">
  <ul id="minimenu">
    <li>
      <strong>levels: </strong>
    </li>
    <li>
      <a href="level1.html">Level 1</a>
    </li>
    <li>
      <a href="level2.html">Level 2</a>
    </li>
    <li>
      <a href="level3.html" class="disabled">Level 3</a>
    </li>
  </ul>
</div>

V1
http://codepen.io/CesarCEARA/pen/LxLyqq http://codepen.io/CesarCEARA/pen/LxLyqq
Regards,
Cesar Barros

