Changing the font size.
HTML you use to 'mark up' plain text. It tells the text what its purpose should be : an unordered / ordered list, a paragraph etc.
CSS is for formatting that marked up content. With CSS, you can control the colour, font, the size of text, the spacing between elements, how elements are positioned and laid out, what background colours are to be used, different displays for different devices and screen sizes and more.
Think of HTML as the abstract text and images behind a web page and CSS as the page that actually gets displayed.
