flex-grow

https://webref.ru/css/flex-grow

by Варлам Ерофеич

HTML

<form action="handler.php">
   <p>
    <label>Имя:</label>
    <input name="name" id="name">
   </p> 
   <p>
    <label>Какая у вас операционка?:</label>
    <select name="os">
     <option value="1">Windows</option>
     <option value="2">Linux</option>
     <option value="3">Mac OS</option>
    </select> 
   </p> 
   <p><button>Отправить</button></p>
  </form>

CSS

body {background: 
  radial-gradient(black 5%, transparent 20%) 0 0,
  radial-gradient(black 5%, transparent 20%) 4px 4px,
  radial-gradient(rgba(255,255,255,.1) 15%, transparent 20%) 1px 1px,
  radial-gradient(rgba(255,255,255,.1) 15%, transparent 20%) 5px 5px;
  background-color:#243844;
  background-size:8px 8px;
  }
   form {
    display: inline-block;
    min-width: 300px;
    margin: auto;
    padding:5px;
    background-color:#def;
   }
   p {
    display: flex;
   }
   label {
    margin-right: 10px;
   }
   input, select {
    flex-grow: 1;
   }