In almost any web browser you can make text bigger by hitting Command (Mac) or Control (Windows) + = (the key with the plus sign). Want the text smaller? Hit Command/Control + - (the key with the minus sign). It’s also easy to go back to the default text size by hitting Command/Control + 0 (the zero key).
Depending on how the web page you’re viewing was designed, the layout and other elements on the page may scale up and down with the text, the text alone may scale, or the design could break entirely. If you make web pages, it’s important to realize that people can and do scale text up and down. Make sure your site works well when they do!