What causes the flash of grey when I click on an input on my iPhone?

/* Ignore this */

* {
  margin: 0;
  padding: 0;
}

/* Width part */

#first {
  display: block;
  background: red;
}

#second {
  display: inline-block;
  background: red;
}

<p id="first">This has a 100% width</p>

<br>

<p id="second">This text has a background color based on the texts lenght</p>

input {
  display: inline-block;
}

<input type="checkbox">

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Why do the inputs flash?</title>

  <style type="text/css">
    input {
      display: block;
      border-width: 0 0 1px 0;
      border-radius: 0px;
    }

  </style>
</head>
<body>
  <div class="i-should-be-full-width">
    <input type="radio" name="mode" id="mode1" value="mode1">
  </div>
  <label for="mode1">Mode 1</label>

  <input type="radio" name="mode" id="mode2" value="mode2">
  <label for="mode2">Mode 2</label>

  <input type="radio" name="mode" id="mode3" value="mode3">
  <label for="mode3">Mode 3</label>

  <label for="hello">
    <input checked type="checkbox" id="hello">
    <span class="toggle">
      <span class="switch"></span>
    </span>
    <span class="label">Check me</span>
  </label>
  
  <form action="">
    <label for="text-input">
      Enter text
      <input type="text">
    </label>

    <label for="email-iput">
      Enter email here
      <input type="email">
    </label>
  </form>
</body>
</html>