CSS

Description

A.k.a Cascading Style Sheet
CSS is the styling language of websites. Styling includes properties like

display
position
color
transformation
font-size
...

The properties can be applied to HTML tags.

Example

<!DOCTYPE html>
<html>
  <head>
    <title>Page Title</title>
    <style>
      .i-am-red {
        color: red;
      }

      .i-am-rotated {
        transform: rotate(90deg);
      }

      .i-am-not-visible {
        display: none;
      }
    </style>
  </head>
  <body>

    <h1 class="i-am-red">This is a Heading</h1>
    <p class="i-am-rotated">This is a paragraph.</p>
    <div class="i-am-not-visible">
      This is a box
      <div>Inside another box</div>
    </div>

  </body>
</html>

Learn more

Basics: https://www.w3schools.com/css/css_intro.asp
Basics (de): https://www.mediaevent.de/css/

PS

You can also style SVGs with CSS!