Users can configure what size font they want their browser to use, meaning what is a minuscule font with one user's browser might be gigantic with another user's browser.
Users can toggle other display options, such as deciding to hide graphics.
Browsers on different operating systems may render form elements such as radio buttons and check boxes differently, using the interface controls provided by the operating system. Screen sizes vary, causing a page's margins to be different from one user to the next. Some browsers support languages like Java differently than others, meaning that sites that incorporate such technology will appear differently from browser to browser, and may not even work in some instances. Therefore, loading this page in Microsoft Internet Explorer results in several Javascript errors, and the page doesn't do any of the things it was designed for. As a result, you are never entirely sure how your web page will look to another user.