Whitespace makes code easier to read by increasing (or decreasing) the spacing between lines of code. To make the structure of code easier to read, web developers often use indentation.
The World Wide Web Consortium (W3C) is responsible for maintaining the style standards of HTML. At the time of writing, the W3C recommends 2 spaces of indentation when writing HTML code. Indentation is intended for elements nested within other elements.
In the example above, the list items are indented with two spaces. The spaces are inserted using the spacebar on your keyboard. Unless your text editor has been configured properly, the "TAB" key on your keyboard should not be used for indentation.