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units in css and z-index

px (pixels) : Fixed-size units, known as pixels, do not adjust to screen size.

em : Relative to the font size of its parent. For example, if the parent's font size is 16px, 1em is equal to 16px.

rem : rem means Relative to its root element.

vh (viewport height) : A percentage of the viewport's height.

vw (viewport width) : A percentage of the viewport's width.

% (percentage) : Represents a portion of a whole, often relative to a parent element's size.

Example :

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Units</title>
    <style>
      body {
        font-size: 16px;
      }

      .btn {
        width: 2rem;
      }
    </style>
  </head>
  <body>
    <button class="btn">Click Me!</button>
  </body>
</html>

Output :

screenshot-1

Example Explanation :

In the above example, the body's font-size is 16px. The width of a button element with class .btn is twice the size of its parent element.

Example :

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Units</title>
    <style>
      body {
        font-size: 16px;
      }

      .btn {
        width: 2rem;
      }

      .btn-1 {
        width: 32px;
      }
    </style>
  </head>
  <body>
    <button class="btn">Click Me!</button>
    <br /><br /><br />
    <button class="btn-1">Click Me!</button>
  </body>
</html>

Output :

screenshot-2

Example :

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Units</title>
    <style>
      * {
        margin: 0;
        padding: 0;
      }

      .container {
        background-color: tomato;
        height: 100vh;
      }
    </style>
  </head>
  <body>
    <div class="container">This is container.</div>
  </body>
</html>

Output :

screenshot-3

Example Explanation :

In the above example, there is a div tag with a .container class that has css properties for background color and 100vh height, which fills the entire viewport.

Example :

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Units</title>
    <style>
      * {
        margin: 0;
        padding: 0;
      }

      .container {
        background-color: tomato;
        height: 50vh;
        width: 50vw;
      }
    </style>
  </head>
  <body>
    <div class="container">This is container.</div>
  </body>
</html>

Output :

screenshot-4

Example :

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Units</title>
    <style>
      .container {
        background-color: tomato;
        height: 100%;
        width: 50%;
      }

      .btn {
        width: 100%;
      }
    </style>
  </head>
  <body>
    <div class="container">
      <h1>I am container.</h1>
      <button class="btn">Hello</button>
    </div>
  </body>
</html>

Output :

screenshot-5

z-index​

In CSS, the z-index property controls the stacking order of elements on a webpage. Higher values position elements on top of lower ones, creating a layered effect, similar to stacking cards.

Example :

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Z-index</title>
    <style>
      * {
        margin: 0;
        padding: 0;
      }
      .container {
        background-color: tomato;
        height: 500px;
        width: 500px;
      }

      .btn-1 {
        position: absolute;
        left: 10px;
        top: 10px;
        background-color: aqua;
        width: 100px;
        height: 50px;
        z-index: 2;
      }

      .btn-2 {
        position: absolute;
        left: 30px;
        top: 30px;
        background-color: bisque;
        width: 100px;
        height: 50px;
        z-index: 3;
      }
    </style>
  </head>
  <body>
    <div class="container">
      <button class="btn-1">Button-1</button>
      <button class="btn-2">Button-2</button>
    </div>
  </body>
</html>

Output :

screenshot-6

Example :

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Z-index</title>
    <style>
      * {
        margin: 0;
        padding: 0;
      }
      .container {
        background-color: tomato;
        height: 500px;
        width: 500px;
      }

      .btn-1 {
        position: absolute;
        left: 10px;
        top: 10px;
        background-color: aqua;
        width: 100px;
        height: 50px;
        z-index: 1;
      }

      .btn-2 {
        position: absolute;
        left: 30px;
        top: 30px;
        background-color: bisque;
        width: 100px;
        height: 50px;
        z-index: 2;
      }
    </style>
  </head>
  <body>
    <div class="container">
      <button class="btn-1">Button-1</button>
      <button class="btn-2">Button-2</button>
    </div>
  </body>
</html>

Output :

screenshot-7